Well, I have yet to hear this. But it’s a rather curious statement for a couple of reasons:
First, what does the “will” indicate in that statement? Either homosexuality does or doesn’t have a net affect on population. “Will” indicates that it has not yet had an impact on population, but in the future it will. Odd.
Second, I’ll assume that someone making this statement sees population decline (or a slow down in population increase) as a bad thing. I’m not sure why this would be. We have (or will have) an overpopulation problem.
Third, I’m not sure how any of this is relevant to those who wish to restrict the rights of homosexuals.