The first sexual thought I had (that I recall) was when I was five (I had already begun masturbating). Even then, even though I didn't know what those thoughts were, I knew it was wrong and disgusting so I stopped thinking about those things until puberty. Now I'm in my mid-20's and constantly plagued by sexual thoughts. I say plagued because it just makes me want it more and I've never had it. And it's kind of wrong to be thinking about someone specific I know in real life.