Evolution does not take into account outside sources, technological advancements etc. Evolution, for better or worse, drives us to procreate. You have to look at evolution at its most basic: survival of the species. Evolution does not know how big our planet is or whether overpopulation would be an issue. It simply drives us to procreate.
So I have to adopt your somewhat fictitious description of homosexuality so I can maybe entertain the idea that somehow homosexuality is undesirable from and evolutionary perspective?
I'm not sure what you think evolution is, but evolution is simply change in the genetic make-up of a population over time (i.e. successive generations). A species, and therefore its evolution, are affected by all the things that you're trying to rule out in your above post: available resources in the environment, outside influences, e.g. catastrophes that cause major extinction events, and so on.
The title of your thread is "Homosexuality and evolution" You admit that homosexuality is not a genetic trait, and therefore you have to admit that it cannot be operated upon by evolution, making this whole thread pointless, But no matter.
Your point was that homosexuality does not serve the species, and if anything it clearly does serve the species by reducing overpopulation, IF your claim that homosexuals reproduce less is correct.
If your claim is incorrect and homosexuals do reproduce as much as heterosexuals (through whatever means), then your whole argument, which relies on the reproduction assumption, is invalid. No matter how you look at it, there isn't anything to argue here.