If homosexuals are less likely to produce offspring, or likely to produce fewer offspring, then in a world where overpopulation is a major threat to our species, then if anything, homosexuality is something that really serves our species. not the other way around.
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.