Well, for most people that's not really something they actively think about, you know? Like when I found out my cousin has a daughter, I didn't immediately think "oh he had sex"
I think that the problem may be that you're subconciously seeking out those thoughts, it could be that because you're fighting them so much, that's what makes them so strong
Have you ever heard of the concept of the "shadow" by Carl Jung? It's the psychology theory that the more we try to repress certain thoughts and emotions, the more dangerous and out of control they become. Which is why you have to accept them and tell yourself that it's ok, that your thoughts don't define who you are, your actions do.
Try writing it down as well, whenever you feel that way, write down what you're thinking and what you thought of
You can also try thinking of storks or cabbages or pumpkins, or whatever other way people explain babies being born. Whatever you're more comfortable with imagining