Hmm. The original post wasn't clear: This is someone you don't know, but it doesn't say it was never discussed prior, and I agree with all the above posters that that changes the whole thing.

I would think that most people in this type of relationship at one time or another would have had a discussion about threesomes. M and I have discussed it. I would be open IF: there was 100% certainty that the person was healthy; this third person would be able to keep their mouth shut after, and they understood that the things we would do would not be your run of the mill stuff. Of course, this narrows the possibility of it ever happening down to nothing.

But if after those ground rules had been covered, he shows up out of the blue with this stranger, then let's go! However, if we hadn't discussed it in detail at some prior time, and he just assumed, then showed up with a stranger....hoo boy. Trouble.