There must be some innate aspect of men -hormonal or otherwise, that means gay guys are less likely to have a strict monogamous lifelong relationship. — Benj96
An evolutionary aspect is that males in many species have lower reproduction costs than females. Caring for offspring may require many months of effort. Another evolutionary aspect is that in many species males can mate with successive partners, while females tend to cease mating after fertilization. Our primate relatives may be quite disreputably (or admirably) promiscuous, depending on one's POV.
Whatever evolution has to say, humans need (and like) to have lasting, loving relationships with others. That said, humans also like to have sexual adventures and novel excitements--both, if possible. Social norms, mores, customs, rules, and regulations try to apply the brakes on said sexual adventures and novel excitements, for the sake of the children, motherhood, community, and the State. For the most part, all that works reasonably well.
In modern societies (post 1875, say) where immigration, lots of population movement, new technologies, social disruption, and so on resulted in looser societal controls and more atomization, some individuals found themselves outside settled community and were thus able to pursue whatever desires they had. Rates of prostitution rose rapidly. The notorious saloons of the prohibition movement were another result.
Unfettered social rules made it possible for gay men to meet and mate outside the palisades of social control. A subculture of promiscuous sex followed, and flourished when and where possible. "Possible" was governed by how much effort a given police force was willing to expend on suppressing this sub-culture. So, this subculture was not usually wide open and public, but kept a low profile and carried on its activities wherever it was possible.
Urban environments are favored places for cultural innovation, invention, sexual adventures, and novel experiences. So with gay male culture. Besides, there aren't enough gay men per square mile of prairie, farm, and forest to provide for partners, let alone promiscuity. Consequently, gay men tend to move toward larger urban centers -- along with any other mobile people who want more opportunities to succeed in life.
I believe a fixed percentage of men are born gay. It's not a large cohort--maybe 2% or 3%. Nature does its part, but then culture decides how gayness will or will not be expressed. Many people in Uganda, for example, believe that homosexuality does not exist there. In that context, expression of gayness is going to be very muted, for self-protection. Kampala, Uganda -- a city of 1.5 million, might offer more opportunity for expression. However, in a country where homosexual activity is a newly defined capital crime (life imprisonment or death for "aggravated homosexuality"), it won't be very open there, either.
There are costs to living outside the security of marriage and family, but there are certainly costs to living within marriage, as well.