I understand your point, and I feel sorry that you have to deal with something that is making you feel insecure about him. However, think that he had the courage to confess you this, he could have kept it like a secret and then all would be fine. For me, that's admirable and requires integrity, love and as I said, courage.
Independent of he saw other girls through this couch situation, he is with you now, how does he treat you? because this day is better to trust the actions than the words.
As I see it you have two choices, you can accept the  fact that your man is not perfect and let go out this thought, or you can talk to him and ask him directly, that will help you feel better but you may hear something that you don't like or you may not, and he will feel insecure and prevented to share this kind of things about his past with you in the future because he doesn't want you to get upset.
What I would do is, I would tell him that I admire his honesty but I prefer that he doesn't share with me this kind of details from his past since I can't change the facts and I can't stop of thinking about it later.