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.
I 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.

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 to other. 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, you can ask them 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.