Confusion is understandable, but I don't think you need to feel guilty. You haven't done anything wrong.

I'm not Christian, exactly, but my mother's side of the family are Baptists. My father raised me on [...] some flavor of Paganism but I've been to my grandparents' church enough to understand the culture and belief system.

I know that the assembly felt like it was promoting premarital sex, but they have such a large audience and I think they just want to make sure everyone is safe if they do choose to engage in that. It's better to be informed and prepared, I think? For if or when it does happen? But it's still your choice to make. The choice to abstain is a valid one and nobody will think less of you for sticking by your beliefs.

I don't think it should have been mandatory, but I understand why they made us attend.