First, it is very narrow to look at this issue solely from the point of population – in fact it is such a tangential point compared to the social, moral, religious, family, health, freedom etc. aspects that it always strikes me as a red herring type of argument.
Second, it is a mistaken assumption to think that homosexuals do not increase population. Or haven’t you heard of all the cases of homosexual couples relying on artificial insemination or surrogate parents in order to have a child that is genetically related to at least one of them? (Funny isn’t it… It’s like there is some undeniable natural instinct to pass on their genes regardless of their non reproductively sexual preference…)
Third, Malthusian doomsday prophecies of overpopulation are SOOOO last decade. We are feeding more and more people with less and less land area and effort. The problem of hunger is one of overconsumption by some and poor distribution to the others, as well local socioeconomic and political factors, not one of overall global lack of food. In fact, population decline is a major issue today – of the entire developed world, only America has a (barely) replacement level birthrate. All the others (Europe, Japan, Russia etc) are having so few children that if the trend continues, they will all simply all die out as a nation.