you are imagining these things, purely because you don't want to, the reason you see you father doing sexual things isn't because that's what you want to imagine, it is simply the worse thing you could think about imagining. in short you have developed a fear of your own imagination. this fear is causing you to imagine terrible things. just like if i told you now not to imagine a dead puppy, it the very thought of not imagining it causes you to do it.
unfortunately there is not much you can do about it for now, just try to relax more.
unfortunately there is not much you can do about it for now, just try to relax more.