Is this really matter? If it is bothering you then I suggest you guys don't ask your gf or wife this question. We are very open to each other, my gf has lots of man before me and she told me that she had more than 50 man (including one night stand) before me, but can't remember the actual number. I also have about 20 ladies before I'm in relationship with her.
For me, I don't care this too much and I think the most important point is the love between two of you. My opinion for a good relationship is do not care about too much the past of your 2nd half, once two of you are together then should look forward and try your best to treat, love and take care of your 2nd half (also imply to lady as well).