pringlecat "A married former colleague of mine with 2 young DC (I don't think he's at a sporting event today!) does flirt with me all the time. We both see it as harmless banter because we've drawn our line and we would never step over it."
I wonder if his wife sees this as harmless banter, or whether you are in danger of leading to the breakup of his marriage and him living without his kids? I know you may not see it this way but do you know his wife is not bothered by this? Has she said so to you or have you assumed this is OK? Have you assumed when he said it was OK that he was right?
"He loves his DW and their 2 DC dearly (he's always wanted to be a hands-on dad)" Yes he just doesn't respect his wife enough not to flirt with you, not very nice really.
Maybe he does fancy you more than you know, not nice for him to rub his wife's face in this, or does she not even know about the flirting?
"...and we have completely different life goals. The only reason why he does feel able to flirt with me is because it is a definite fact that nothing will ever happen."
This is a very strange statement to make, so clinical, like some sort of game you've worked out the rules to. So he flirts, which is usually a signal of some sexual interest, but you can trust yourself because of some pre-arranged (unspoken) rules?
That either sounds like a very sterile kind of flirting or two people fooling themselves.
"... he is devoted to his young family."
I think you might try having your friendship on to a non flirty footing, one that doesn't involve flirty banter, when his young children are teenagers they will probably find it excruciatingly embarrassing that their dad flirts with his old friend, I know I would have.
I am not being mean but I know if my husband or my dad behaved like this in public it would be awful and if it is all in private then the danger of something happening is more real.
I have a friend who loved me dearly, and I him, we never got together and married other people. I;d never flirt with him out of respect for my husband, his wife and our 5 children.
I am not meaning to be mean, honest,
I just wonder if you have thought about it from his wife's point of view.