Just because one could stick his **** wherever he wants, doesn’t mean you will.This is hilarious because the whole premise is wrong. Please go back and study biological evolution.
Homosexuals can and do reproduce. Just because many don't doesn't mean they can't, and this is an important distinction in evolutionary biology. Homosexuals are fertile, therefore they serve a natural selection function. They have had children throughout history. Even in the modern world, it's not uncommon for a person to take an opposite sex partner to fulfill familial duties and have a same-sex lover on the side. The notion that homosexuals can't have opposite-sex sex and heterosexuals can't have homosexual sex is a modern social invention.
Besides that, heterosexuals produce homosexuals even in the complete absence of homosexuals. The parents are not secretly homosexual.
Homosexuality seems to be tied to female fecundity. The more fertile a woman is, the more likely she is to produce homosexual offspring. There is likely an altruistic advantage to this because an offspring that is less likely to reproduce is more likely to aid the fecundity of females in the community. In other words, homosexuals are biological altruists because they aid the fitness advantage of other reproductive humans at the cost to their own reproduction. It doesn't mean they don't reproduce though, it just means they are less likely to. Nature is cool like that. Even in those less likely to reproduce, they are not sterile and therefore can still contribute to natural selection.
Go back and learn some biology!
Homosexuals can reproduce but the absence of desire for the opposite sex would lead to sub-replacement fertility. The OP isn’t wrong - homosexuality breeds itself out of existence.
You’re right that birth order has a positive correlation with odds of being gay but the effect is rather weak.
The rest of what you wrote about biological altruism and “nature is cool like that” is just speculation.