I've had experiences of ALL of the things posters have mentioned, some good posts here.
conclusion:
-the hive mind is ace, a bit like a big goddess
- also i've got round a lot.
I think being very self-aware is the key? so honestly ask oneself "what are everyone's motivations here, is there anything instinctively off with this dynamic, is the other person assuming things and if so can they be diplomatically forestalled?"
When i've had issues with boundaries it because I haven't really, really thought about things - or haven't WANTED to - and have done the 'buried head in sand' thing emotionally?
It can be very lonely socially surviving as a woman (human) and I think sometimes "a guy friend who I know secretly has a crush on me" is an 'easy social option' for a woman.
so for example, I made friends with a single guy, a bit geeky, I sort of secretly 'knew' he had a crush on me, but didn't really confront the issue and made some shite excuse about "not wanting to date due to depression"? (which was a lie, as I was interested in other guys, just not him)
which I do think I could have done better, as I might have "owned" the situation socially earlier, by making it VERY clear that I wasn't interested in him regardless - then I might even have helped him with his dating efforts, rather than letting things get to the stage where he was a bit pissy and resentful.
OP I do think its a very reasonable approach, transforming your friend into a "couple to couple friendship".
I'm a bit puzzled by your partners response, can your partner not come up with an actual reason as to why he feels uncomfortable? I mean "texts too late at night" is a reason. "He said X" is a reason. But "I don't like him" doesn't seem that good enough? how is he with your female friends?