ok, i'll bite.
i guess my question would be how you and your partner draw your own personal lines on your exclusivity. it sounds like you made a new friend, felt the crush of friendly infatuation, knowing it couldn't go anywhere. that's actually nice. your partner can do many things, but he can't be brand new again, and you sound like you're conscious and handling it like an adult.
what i'm not clear on is why it matters if your new friend and his partner have a 3rd occasionally. it's not for me, personally, but for some people it seems to work out well (within clear boundaries). if you were never going to cross that line, isn't it irrelevant? if you had considered crossing that line, wouldn't you need to know what chose clear boundaries are? maybe they are not clear, in which case, i'd steer super clear of them. but if they are clear (like maybe they just want to watch...or something)
can you not be friends with someone just because there is a vibe between the 2 of you? are you afraid you would act on it? that he would?
i'm not sure i see the point of giving up a friend just because you had what is effectively a crush.
someone mentioned the 80/20 rule and it sounds like an example of this, but you're smart enough to realize where your 80 is