I wouldnít even ask. Everyone has some baggage of some sort. If she chose to tell me it would be to ease her conscious and to let me know where she came from. Where you came from isnít as important as how you got to where you are, what you learned on the way and most important, where you are now. Iím told when we repent, truly repent, we are forgiven and it is forgotten by our Father in Heaven. We however, do not forget least we make the same mistake again.
When I was dating my wife she told me one night that there were things in her past she was ashamed of. I told her that wasnít important to me; (her past) because I could see who she was and I could see what and who she could be. And that is why I wanted her in my life. I saw a home centered on the Savior, a home of love and learning, one where the outside world would not be the most important function.
I have no idea what she might have said that night and she has never disappointed me. She has filled my life with more than I could ever imagine. If YOU feel you need to share something, then do. If he changes his mind about you then he never really saw you in the first place. Sad as it might be, itís better to know about him up front. But I don't think it's important.