The first answer I would give is male pride. The second is insecurity.
Traditionally males have been the breadwinner.... anyway the upshot is that you probably won't give much thought to this if your relationship doesn't work out and you find another woman who happens to be a virgin. It can only be your first time once, and I'm sure your girlfriend would take back those other guys if she could, but it's done and she can't change that. If you love her you will understand that her telling you about her other men is her way of trying to make this the first time for her too.
The insecurity bit is something you have to get past yourself. Worrying about if others have been better, bigger, or longer does no good for either of you. She is with you. She loves you. Please don't ever try to use she past against her in an argument, as that is the lowest possible thing to do and will tear her heart apart after she has trusted you with such an intimate confidence. You don't know how lucky you are to have her.