Or you could just tell the truth that no science has yet produced a study that has confirmed a reason/purpose/cause for homosexuality and until someone can convince you otherwise, it is a human defect in the sexual drive along the lines of other sexual deviations and gender reversal issues.

Start there and there's no need to get into sex, data, children etc. unless somebody tries to feign blindness to the obvious.

That's usually when the "Pro-gay" crap starts with the "legal","in their homes", "Not hurting anyone", "what's it to you", and how could you forget "homophobe!", etc.

Whatever.