Men and women are different. These differences are significant to the function of social groups. There are many examples of social groups, especially in politics and hobbies, ruined and derailed by inter-sexual drama. Purposeful groups should therefore adopt rules to eliminate sociosexual drama and improve group cohesion.
Here is a set of rules that we use in our groups for this purpose:
Brotherhood. The group shall be composed exclusively of men.
No Ex-Girlfriends. No man in the group shall be in contact with any member's ex girlfriend.
Monoandry. No man in the group shall flirt with another's girlfriend, nor allow any flirting between girlfriends and any other man.
Monogyny. No man in the group shall pursue multiple women at the same time or in quick succession, especially not to the exclusion of the other men in the group.
To us "enlightened" modern folk, such rules sound archaic, bizzare, and positively illiberal. Why should we be so controlling? Why can't we just let it all be cool, man? But to nearly everyone else in history, they are so obvious as to not be worth mentioning. It turns out that everyone else is right on this one, and the modern liberal is wrong; there are very good reasons to enforce rules like these in male-dominated groups. Let's explore the details:
This rule is a formalization of a biological and social reality, that relationships between men, especially in a group, are of different character than relationships between men and women or in mixed groups. Men in groups generally cooperate and focus on the purpose of the group, conflicts being about hierarchy, which is usually resolved without conflict. Men, male hominids, are quite literally built for this - to cooperate intelligently on purposeful ends to mutual advantage. It is our evolutionary niche. But when women are introduced to a group, the men are instinctively driven to pursue her attention, and start seeing the other men as rivals. The more they pursue her attention, the more they are distracted from the purpose of the group, and the more solidarity with the other group members is broken. And they can no longer speak as freely and openly. This becomes a source of conflict, drama, and dysfunction. Best to rule it out entirely by having an all-male group.
There are multiple reasons for this rule:
Women who hop between multiple men in a group end up with too much leverage and social power over the group, and become the center of drama, while contributing little or nothing to the group. A rule of no contact with ex-girlfriends makes this source of drama and subversion impossible.
Potential loss of social contact with the entire group makes the decision to terminate a relationship more weighty for girlfriends, and thus increases relationship stability, which allows more group attention to be focused on ordered ends.
A girl switching her boyfriend is always psychologically interpreted as an offence by the new boyfriend against the old, despite the usual insistences that the relationship is over and that it's cool. Such jealousy and bitterness are harmful to group cohesion, so such situations should be made impossible by not even being in contact with ex-girlfriends.
Men want secure exclusive access to their women, and want other men to respect that. When another man starts homewrecking, it creates conflict and undermines the group. This applies to past relationships, as above, and it applies double to current relationships. In cold economic terms, Monoandry dictates property rights in women. You wouldn't steal a brother's stuff, so why would you steal his girl?
Failure to enforce this rule internally destroys men's groups.
This rule is the most subtle. Why should men limit themselves to one girlfriend? Isn't that just a handicap?
The major problem with polygyny for brotherhoods is that the most dominant, attractive, and sex-obsessed men in the group will spend their time courting all the women in the local social scene, to the detriment of their focus on group matters, and to the detriment of the other men. In economic terms, the local social pool of available women form a limited supply. Allowing full competition drives up the "price" of the women's affections so that only the most gifted men can afford exclusive relationships with them, and only at great cost of effort and attention, which attention could have been spent on group matters. The "price" competition naturally creates conflict between men in the group, undermining solidarity and cooperation, besides leaving less attention for the group. Monogyny, on the other hand, implements rationing, one per man, which as usual drives down the "price" to something most of the men in the group can afford, and results in a lower collective "price" being paid by the group for their relationships. This allows men to invest in group matters rather than fighting over women. It also shifts the focus to careful choice of higher-quality wife-material women, rather than easy women.
There are also two good non brotherhood-related reasons for monogyny:
Investment in civilization: The above argument does not apply to the extent that the available women are not a shared local pool, but fished from the general "ocean". In that case, it might seem that men should scoop up as many girlfriends as they can get. In a case of general sexual anarchy, this is a decent objection for those few men who will be doing the scooping, but in conditions of civilization, monogyny will be enforced generally for the other reasons here. It is better to pay the relatively small marginal cost in not being able to have multiple girlfriends, and excluding those men who insist, for the smooth compatability with, and the push towards, beneficial civilization-wide sexual regulation. If civilization has any hope, you will have to eventually anyways.
Negotiating leverage with women: As a sole girlfriend will recieve more attention from a monogynous man, the relationship will be worth more to her so she will be happier to cooperate in other ways. For example, she will be less inclined to put herself in positions where her fidelity is at risk if her main relationship is more rewarding. Harem women get less individual attention, and thus have less incentive to cooperate with the relationship. For this reason, polygynous societies have the classic problem of harem security, and need to place more extreme controls on women, with eunuch harem guards, escorts, witnesses, burkas, and so on. This argument is not to be confused with unconditional niceness to a woman somehow making her happier to cooperate. Game theory doesn't work like that; this argument only works when the increased value of a monogynous relationship will be taken away by breaking up with her if she defects.
As a whole system
Besides the justification of the individual rules, the rules as a whole have substantial benefits:
Group Cooperation: These rules eliminate most internal sociosexual conflict, and many sources of subversion, a much stronger and more cooperative male group that is able to coordinate on purposeful things. Thus, adopting these rules makes a group much better at accomplishing the goals that it cares about.
Even directly in the realm of romance, prospects are better: instead of fighting internally over a few women with no solid resolution, that conflict is quickly and permanently resolved, and the only option is to cooperate to meet and court more women. A powerful and coordinated group, should it turn its attention to romantic ends, or even just securing existing relationships against advances by outside men, will do much better than any lone man. No one is as "alpha" as a cohesive brotherhood.
Compatibility: Finally, these rules, despite seeming rather extreme to the modern mind, are the general tradition in our european-derived society and organizations. They are thus compatible with all the other subtle rules we have, and will not clash with our general community's expectations. They are also thus well tested, and will not do anything unexpected or blow up in new ways.
Unfortunately, these rules are much discouraged these days, and may become illegal at some point, as the cohesive male groups they form are difficult for the modern state to control politically. Male groups are not inherently a defection against the rest of society, but they only really fit within an organically ordered society in which the interests of the state are aligned with the interests of the people, because they tend to become locally more powerful than the far-away state. The state no longer understands how to organically align everyone's interests under ordered leadership, so it is opposed to powerful intermediate groups like brotherhoods. This is, incidentally, exactly why these rules seem extreme to the modern mind; everything truly dangerous to state power eventually has public perception turned against it by the media allies of the state. In my opinion, though, a defective political system is not a valid reason to throw out perfectly good social technology, so I'm ignoring this consideration.
The first and most obvious place this needs to be applied is male-dominated hobby groups around politics, video games, fandom, etc. These groups are notorious for being full of men who get excited at the idea of women joining in their hobby, who then have their groups torn apart by socio-sexual drama, not to mention political drama. It keeps happening because there is social and political pressure to include women, and because the young men involved are thirsty for female contact. These rules allow a principled stand against this harmful phenomenon.
Another important place to apply this is in semi-casual friend groups. The temptation to include a girl who is like "one of the guys" is constant, but it always makes a mess. Better to leave her with her girlfriends and be explicitly a brotherhood. Monogyny also needs reinforcing in these groups, especially against the "serial monogamy" failure mode. For example, I had a friend a few years ago who was always single, but very charming. He would always swoop in and court the prettiest girls, to the exclusion of the other guys, but would never settle down and give anyone else a turn either. This was socially corrosive.
These rules eliminate nearly all sociosexual drama, but without adding too much complexity or onerous restrictions above common default "dating" practice. The version presented here is not framed in terms of marriage, but tradional marriage clearly fits the rules, and and does so more or less for the reasons listed here. We can start to see how to reconstruct much of traditional morality from this kind of group conflict-prevention reasoning.
These rules are the seed of traditional high-civilization moral culture, but they are not some abstract ideal that would be a nice thing to have in a golden age. They are practical rules that any male group can adopt right now, without other major changes in lifestyle, and without waiting for everybody else, that offer substantial return. If civilization can be recovered from the ashes of a culture that has forgotten all this stuff, it is by application of small practical "modules" of civilization like these rules.
They don't always apply, of course, so we have to use common sense, but all else equal, these rules are superior to any other way of solving the problem.