With more than six billion humans taxing the carrying capacity of the biosphere, procreation is becoming more of a threat than a guarantee to human survival. That makes gay and lesbian relationships more environmentally-friendly than most heterosexual relationships because they don't contribute to population growth. Of course, sexual orientations are fairly inflexible. But according to sex researchers, a large part of the population has intermediate tendencies. That means many individuals whose orientation is predominantly homosexual are capable of having heterosexual relationships even though they are less natural and satisfying.
Bisexuals can go either way with equal satisfaction.
In a homophobic environment, both groups tend to adjust to social norms by marrying the opposite sex and having children. Though less than ideal from the standpoint of personal fulfillment, this outcome is useful to society when its continued existence is threatened by insufficient numbers. The least coercive option is therefore to make it easier for persons who are bisexual or predominantly but not exclusively homosexual to enter into stable homosexual relationships. Being able to marry would also enable them to more easily adopt the unwanted children of heterosexuals. It may be objected that most of the world's population growth is occurring among the poorer nations of Latin America, Asia and Africa, precisely the regions that remain most homophobic.