I have to ask, have you ever seen anyone let someone down more definitively? I never have. We have been actively swinging for 6 or 7 years and in all instances that I have been involved in or have witnessed, it was done just the way the OP described what happened with them.
Frankly, when we are busy trying to set something up with someone, the last thing on my mind is to go to someone we had been previously visiting with, but had made no commitment to, and tell them we aren't interested in hooking up with them that night. In fact, to do that would seem extremely awkward to me, as it demands I first make the assumption that their was some implied commitment with them to begin with.
On the contrary, I don't think this couple was being immature, unless the OP left something out. While it sometimes sucks to be the one passed up on, this is the way it is normally done. As I say, unless the OP left out a point when one couple or the other asked to hook up for play that day, I don't see that their is any better way the other couple could have handled it, and their is surely no indication that they were some how obligated to handle it any differently than they did.