I think you might need to go speak with a therapist to see if you can work through these issues and find out why you're having this problem. *edited* Do you have an underlying issue that makes you feel unworthy of being loved by a good man? In general, I'd say it isn't unhealthy to have images of having sex with someone else enter into your mind. In fact, that's probably VERY normal as long as you don't act on it. Fantasies are just a way for our minds to think, cope and have a little excitement. It isn't necessarily bad unless it causes you a lot of undue stress, and it obviously is. Please do go find someone to talk to so you can get to feeling better :)