Interesting post.
One thing I've learned is that we can never know what's going on in someone else's mind. This goes doubly for swinging: we never know what's happening in someone else's relationship.
It's quite possible that your friend had an experience that has nothing to do with you and your wife, where the other husband didn't like him kissing his wife that much. I've definitely had that happen: I was a "cuddle kissy bunny" (other wife's expression) with a playmate. It turns out that he doesn't get as much kissing and cuddling from her as he would like, and she didn't like seeing him be quite so affectionate with me. And then it turned out that my own husband wasn't that keen on seeing that either, when he wasn't getting it from Mrs. Playmate. He waited until our friends had left after spending a solid week with us, to let me know that. Boy, did that annoy me. If he had told me while we were all together, I could have easily backed off a bit.
(Sorry... got sidetracked there for a minute)
My point is that it might have been a prior experience that is leading your friend to ask you that question. Or it could be any of the possible reasons you listed in your OP. Or... it could be something else entirely, something neither you nor any of us could guess.
Which is why, if the question strikes you as odd and you have a funny feeling, you should ask him about it directly. It doesn't have to be a confrontation. Just "hey, you asked about x and I was wondering why you were concerned".
I am curious too. I hope you find out and let us all know
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