^^ Go watch that scene with them at the bar, Don was flirting with her and Joan was basically tellin him that she wanted to fuck and didn't care about either of them being married.
"I like being bad and then going home and being good"
"He knows. That's just the way he is. And maybe it's just the way she is"
then offering to drive him home
Mad Men is subtle this ain't reality tv they're not gonna throw it in your face
It was just good flirting, and of course both Don and Joan would be good at flirting and have great chemistry. But at the end of the day, it seemed more like a casual, old colleagues interaction. He was very supportive and kind to her when she needed it, including sending those flowers. Them screwing would accomplish nothing for either of them and be very predictable...which is not Mad Men.