I think OP has gone now.
But I find this topic interesting "the one that got away".
Honestly, I think the whole concept is pretty much nonsense, and am surprised people think its common.
What does that mean "the one that got away". It usually means:
(a) the other person didn't want you! so you soften the blow and see it through rosy tinted spectacles by saying "the one that got away"! OR
(b) the one you didn't have the balls to go for at the time. Usually by passive wasters living in dream land! They can be all "wistful" when it was really their lack of gumption that was the problem. But they don't usually mention that part! Perhaps that self-reflection would be the more relevant and important part.
There are possibly exceptions (think film Casablanca) but for most people its just a natural outcome of rejection, weakness or circumstances neither party decided to try to overcome. To drone wistfully on about the "one that got away" is probably a bit of a rosy tinted spin on the reality of things.
I once was interested in a man who droned on about the one that got away. With nothing else forth coming I though he sounded weak and self-centred, which he probably was. I did think men sufferred from this more though, having more of a "fantasy" view of women than a "real" one.
In the OP's case its a bit harder to judge, because she put the question to him, and as PPs have said put him on the spot unfairly, in a group situation too. So OP has exposed her own carelessness or insecurity.
Now I am older, I would be inclined to ask any romantic interest who started on about the "one who got away" lots of questions. I think, personally, this would result in lots of squirming! 