If you would like a psychological perspective on sex and sexual thoughts, here you go:
In adolescence, the human mind goes through some startling changes along with the body; as the brain continues to physically grow and mature, new neurons and synapses begin to fire, connecting new parts of the brain and creating new sensory pathways, making way for sexual thoughts and impulses.
Sometimes, we have ideas about sex that conflict between how we were taught or what we picked up over the years and how we feel on the inside. This difference, this internal friction, is called "dissonance". Dissonance naturally seeks resolution, as our minds are not very fond of conflicting ideas.
Somewhere along the way, you picked up feelings or ideas that conflict with your sexual impulses and it is making you feel bad, well here is the way it really is in the adult world: you do not ever need forgiveness for your sexual thoughts. Your sexual thoughts are nobody's business but your own, and in that same vein, never justify your desires to anyone, and never be afraid to deny that they turn you on and invigorate you.
There is no "unacceptable" because there is no authority over sexual thoughts, there is no "forgiveness" because you are not doing anything wrong.
So relax! Enjoy your private sexual fantasies, and do not forget that you owe nobody an explanation for how you fantasize about them (unless they are fantasizing about you as well, but that is another conversation entirely).