I am in a mono relationship, me and my boyfriend have been dating for a few months so not a lot of time. My bf had a bit of 'poly mindset' (casual sex with different people while having a 'main' person), but after I said that's not enough for me and I'd like to be exclusive, he agreed. He was surprised that exclusivity also includes no sexting, I found out he did it but we talked it out and I didn't even have to ask him to stop, he said he will stop because it upsets me.

But I think it made me develop a new fetish. I would like him to sext someone while I'm watching (and the other girl is aware of that) or maybe I'd jerk him off while the other girl watches.

How should I put this idea into words? My bf will be surprised since I made it clear I'm not interested in anything else than monogamy. And how do I make it clear that he can only sext other people when I'm there? And that we're staying mono with a little exception to trying this fetish and possible future times we do it if we both like it?

I don't wanna make it look like a business transaction, but I still wanna set my boundaries. I don't even know if it fits calling it "opening" the relationship, it's just like a threesome but online.