Something similar happened to someone I know a few years ago... 8 couples invited by Jenny and Steve (to their new year's eve party,) except the ONE couple Louise and David. Long story short, it turned out that Jenny suspected her husband Steve fancied Louise. (I suspect so too.)
This is one reason why constructing your entire social life around coupledom is always a bit of a hire wire act. I've always been quite suspicious of people who only want to be friends with other couples. For several reasons:
a) In any given friendship group comprised only of couples only some couples will stay the course. If Couple A split up and then John immediately couples up with Helen from next door he's going to become alienated from the group, while he might suddenly find that he suddenly has to become friends with all of Helen's friendship group (who may be suspicious of him).
b) the nature of the dynamics doesn't allow much space for intermingling between couples. If Bob from couple A decides to have a long chat with Sue from couple C at a dinner party without Steve and Anne from couple B could get suspicious and gossip about it to Couple D.
c) It suggests a level of suspicion and mistrust of single people because they are seen to pose a risk to any of the existing couples (not to mention a closed-minded approach to people who aren't in a relationship.)
I was part of a friendship group when I was in my mid-late 20s with a bunch of couples and I was the only single person in the group. I discovered that some of them had taken to having gatherings only as couples which they didn't tell me about but which one later confessed I'd been left out of "so as not to make you feel left out". It was absolutely pathetic, as if I was going to be jealous of people simply based on their relationship status. I dropped most of them after that.
Bringing a mix of other people into the group, including singles, makes for a much more open and relaxed dynamic.