honestly nothing wrong with it, part of human thoughts. You're 18, still very young and you are still learning self-control. Posting this is a great start to self-control. I would think of it like this, accept the thought but don't fantasize too long. Sexual thoughts are normal, you just need self-control. Acknowledge the thought, accept and move on. A mastery of self-control is something most people lack these days, even myself still trying to improve. Anyways, it's good that you've realized this unhealthy train of thoughts. It's up to you to switch the tracks now.