If you're disgusted simply by knowing that another man's penis has been inside a woman at some point in her past, then that's a problem you need to overcome. There's nothing disgusting about that. You've associated some sort of shame or filth with sex. You need to let go of that. Specifically, what about it is disgusting to you? You think she's not clean? Vaginas are not like condoms. They aren't meant to be used once and discarded. It's not like you would think a woman were unclean after sleeping with you, right? It's not like you wouldn't ever have sex with her again, right? What's the difference? Do you touch door handles? Do you use public bathrooms? Those things are dirty. Your girlfriend's vagina is not dirty or disgusting.

I really think you have baggage left over from religious brainwashing about sexuality. If a woman loves you and wants to make love to you, then she isn't in the possession of some past lover. She isn't unclean. She's a person who wants you to touch her and experience physical intimacy with her, and that is a beautiful thing.