If being involved in ity is a sin, then why did God allow it?

I was talking about this with my dad, and he made a pretty convincing argument, but something keeps nagging at me. Yes, the 'bible' says uality or being involved in acts is sinful, then why would he allow this? Now my dad and I are both strong Christians. But he says God didn't "create" gays, he gave us free will and they "chose" to be like that. I think everyone that is up-to-date with the 21st century KNOWS that this is so entirely untrue its insane. Why would anyone chose to be ridiculed and sin and be made fun of? I know I wouldn't. So why, then, is it a sin? I can't fathom it.