There are some problems with having sex with a child that don't just reside in the negative social reaction. Children can be talked into doing something they don't really want to do and then find it revolting for ever after, for example. Sex is not necessarily wonderful to a child even if he seems to consent; in fact, for at least some, it can be horrifying under those conditions. Usually I encourage people to learn to live with their sexual orientations and not worry too much about fantasies that are strictly kept as fantasies. That works best, though, when they have a very clear idea of why they wouldn't want to act out their fantasies in real life.

Most of your great reward will come from not causing harm in real life, not from suppressing your imagination. It's a little difficult to explain, but if you become too attached to your imaginings, then you could indeed be walking in the way of the flesh, becoming the mental version of a porn addict. If you just put up with them matter-of-factly as part of your biology, though, and try to follow the way of the Lord in everything that's under your true control, then you'll be walking in the Spirit as a boylover.