Felicity53, yes, you're on the right track. I had a similar experience and there's no possibility of getting back to that happy friendship, I'm afraid, once that line has been crossed.
I also wondered about possible early dementia. It seemed so completely out of character (and unwise) for him to even suggest a relationship.
Later, I thought that perhaps, secretly, he'd been thinking along those lines for quite some time - and waiting for the right moment to make an approach. I was just horrified, as he's married with a family living nearby.
Yes, I did get an apology, which I accepted, although it had to be understood that the friendship was over - permanently.