In a totally different area, I have felt the way you're describing your guy friend feeling...I've felt that heart wrenching pain when I heard a subject come up in conversation (in our case, my ex was indulging in some very intimate things with another woman - it was something I also was interested in and something they refused to share with me, and that hurt me a lot). I can see this hurting his feelings a whole lot simply because he doesn't understand what it's like to have a fetish and how healthy and normal it is to be a part of a community. To him, he's only seeing it as a sexual act that you're spreading around with whoever (emotionally, if not physically).
Now that being said, it's not healthy or right for him to continue to handle it that way. In my situation I went through utter hell, I gave myself bleeding ulcers over it, I alienated myself from my partner because they couldn't mention the "other woman" or anything connected to her, without me bristling and being cranky the rest of the day. I hurt myself and I hurt my partner, and for what? It didn't change her mind or help the situation, it just made her more secretive.
I am sharing this with you because I'm not by nature a controlling, jealous type, and I want people to know that it can still happen to well-adjusted, open-minded people who just really care about their partner and let that caring manifest in the wrong way. He needs to stop, for both your happiness and his own peace of mind. If you ever need someone else in the community to talk to him, I am more than willing to share my story with him. I know you don't know me and I'm sure there are others who you are more comfy with, but the offer is there. I wouldn't want to see someone else go through (and put their partner through) the same mistake that I made. It is a miserable way to be and it destroys relationships.