The idea that the only natural form of sex is sex in service of procreation must rely on the logical assumption that sex outside the purposes of sexual reproduction is unnatural. Does that logically follow? Are horny young men thinking about being fathers when they're staring at girls in their class?
What a strange non-argument.
Yes, boys lusting after girls in class is ultimately for the intended reason of reproduction. I met my wife lusting for her when I was getting my bachelor's degree. I know you're not black, but what gender are you? It's well down the list of 37 genders, isn't it?