I think the main reason why a traditional heterosexual monogamous relationship works, is a lack of information or possibilities to compare.
Even if you skip the monogamous and just stick to heterosexual, you're usually all on your own. In other words: guys know different girls and may be able to compare or girls know different guys etc.
What you don't know is how you compare to people of the same gender.

Nowadays we (at least in europe) live in a society that seems almost addicted to determinations in that field. Like: "I'm an active homosexual male" or else. That stands against the classic isolation policy.

Personally I think that a great deal of that is group pressure determined, none of my concern and doesn't have a pathological background at all. But some people just feel unhappy with partners of the opposite sex and those are the ones who should have the chance to live the life they have to live.

I'm all for intergenetical same sex marriages cause I love my cats, hehehe