There are, of course, many alternatives to heterosexuality. The ones that most people think of are homosexuality or bisexuality. And then there are other forms of sexuality that most people would regard as perversions, such as necrophilia or frotteurism. But what a lot of people don't think about is asexuality or celibacy, which is practiced by a surprisingly large number of people for a remarkably wide variety of reasons, many of which are not religious. There are even some people who experience atrophy of the sexual organs because they don't use them at all, and there is actually some truth to the adage "use it or lose it."
Well, the argument for alternatives to heterosexuality, including celibacy can be made on the grounds that heterosexuality is destroying the earth's ecosystem, due to overpopulation.
Overabundance of human beings is depleting the oceans of fish, and causing pollution on a massive scale. We are, quite literally destroying the planet because there are too many of us.
Many animal and plant species are becoming extinct, in order to make room for an increasing number of humans.
So being celibate or practicing other forms of sexuality would be a good idea for the long-term health of the planet.