I don't know how I would feel in such a situation.
On the one hand, I understand anyone's need for privacy, and would want to respect that.
On the other hand, I've always talked about our sex lives with my girlfriends. Always got my partners' permissions first, by which I don't mean getting the permission before everything, but getting a general "yes, you can talk about our sex life with your friends". Seamus actually added "that's what girls do anyway, I wouldn't feel right asking you not to".
This being said I don't say 100% of everything, and information is information I volunteer, I'm not answering questions.
So, I would say... if I knew his sex with other partners is a taboo subject... it would make me feel excluded from part of his life as a friend, since I discuss things like that openly with my friends. And as far as talking about sex with me, I actually wish he'd do it more :P He doesn't talk to his friends about that.
On the other hand, I can imagine if I had another partner, I wouldn't go talking about one another... I mean, when I was with both Raga and Seamus, I didn't go comparing notes or anything... still I wasn't really keeping secrets either.
In the end, I don't think I'd wait for him to get home from a date and go all "so what did you do? How was it?" or anything like that, but while talking about various things, I would expect things from his personal sexual experience to come up, as I know mine would come up too. And not just talking with him, talking with her, too, I'd want to be able to share that, it's one thing about a metamour that you can ask about things nobody else would have experience with or get advice from each other or things like that, not just in a sexual context but including in a sexual context.