"but NO ONE, EVER, in my long marriage or during my relationship now, has in any way propositioned me." - No offense, but I doubt this. Now you may not have kept them as friends after, or missed/ignored the setup, but I would place $500 bet that if closely examined, it has happened. You may not have been pressed and it may not have been explicit, but that doesn't mean it didn't occur. I suspect that you know that if your came out of a relationship for an extended period of time, a proposition of some kind would be likely to happen eventually (hence the nearly inevitable).
It's a people thing. It has no bearing on you or really on them. People have their moments, failures of judgment, and misreads. Spend enough time with someone and one of those will occur. Now the context and reaction does say a lot about the people involved. But simply the proposition? That's a part of life.