How much should I know?

I've been with my boyfriend for a couple of years now and we get on fantastically well. Despite a 10 year age gap and the fact that I'm at university, we have a strong, healthy relationship. One thing that bugs me, however, is that I know nothing about his past relationships. The only time I ever expressed curiosity was about five months into the relationship; I asked when he had last had a girlfriend. He answered, but was obviously really tense and then said "You're not one of those girls who wants to know everything about their boyfriend's exes, are you?" I've never brought anything like that up again because he was clearly so uncomfortable about it. Now, however, I am starting to wonder if it's normal/healthy for me to know absolutely nothing about his past relationships. I don't want to know all the ins and outs of it or every gory detail, but I am interested in what he has done with his life and how he came to be who he is today and it seems like there's this massive gap. We're starting to think about our future together and things are getting more and more serious, but I wonder if it's right that I'll never know anything about this person/these people who were obviously an important part of his life? What do you think? I just want to have an open and honest relationship with him, without secrecy and mistrust. How much, if anything, should I know about it?

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