Many are bi, so for them it's a choice on whether or not to act on it. So they project that it's a choice for everyone. And the gays are choosing it because they're lustful or sinful or weak or whatever.
Well, to be fair: using captive children for sex is what Moses commanded.
Numbers 31:17-18
17 Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him.
18 But all the women children, that have not known a man by lying with him, keep alive for yourselves.
If you want to have a bad afternoon, read this report on how the Vatican systematically protected and promoted known rapist to some of thehighest offices.