what is the real cause of my anxiety

I don't know if it is ocd really. I have intrusive sexual thoughts. It will be of women I'm around. The worst part is I would never want to have sex with them because they are friends' girlfriends. I feel it's morally wrong and can't get these thoughts out my head. The guilt makes me feel really anxious. I try to replace those thoughts with that of men because I am not gay. Of course, those thoughts turn sexual in nature also. I can't always control these intrusive thoughts and sometimes question that they must be all true. I am so unsure that moments of clarity are few and far in between.