What is my problem?

I was chatting with this guy who I believed to be 18 (I'm in my 30's) who wanted us to hook up. Turns out, he's only 15. I know legally it's wrong and I could go to jail, but morally, what's the big deal? He propositioned me, not vice versa. And even though we decided not to talk anymore because of the legal consequences (and nothing ever happened), I can't stop thinking about him, about the things he said to me, and what it would be like to be with someone so young and eager. What the hell is wrong with me and how can I get over this???