I think most males think that if they accept to do anything a woman wants, they could get her (or keep her if they're in a relationship).
Even humiliating shit, i.e. femdom
For instance, about polyamorous couples shit, I don't think that the idea often comes from the guy, and I don't think either that the guy has more to win with this crap. (water)
But if a guy has the opportunity to have sex with a woman with the condition of sharing her, many would accept because in their mind, sharing a girl is better than not having a girl at all.
Without taking into account their own dignity.