Thanks for the responses, and the link.
That all makes a lot of sense, and I think my head has cleared a little on the matter now, but it really is just the initial ice breaker that I struggle with. If people ask me questions, I have no problem explaining, but to just begin the talk out of nowhere is difficult for me.
I've considered the phrase "open relationship" before, but it just wouldn't work for me. I don't know if it's got to do with where I live or what, but that term seems to imply to people diminished respect for the partnership - and at the same time that there is a "primary" and everything else is relatively meaningless. Similarly, just casually mentioning my "boyfriend" would only suggest the whole "cheater" connotation.
I guess now that I have broken the ice, it's just a matter of clarifying further, and I guess I'll just have to check with the guy in question if he has any particular questions.
This whole thing is just one big mindfuck sometimes...