@Bklyndd,
truth is... he prob gets a high feeling from whatever that married woman gives him. either boost his low self-esteem or mebbe she's a risky choice, so he proposes to a girl who is a safe bet, and he can play with the risky bet(married woman).
The fact is, the married woman is a fantasy he has in his head. He isn't just sexually involved with her, he's emotionally invested in her too. It isn't love, but he prob thinks it is. Prob same for the woman; not willing to leave the safety of her marriage, but wanting affirmation of some kind and getting it from him.
So it may not make much sense to you, but for him, it allows whatever part of him that she fulfills, that somehow the woman he proposed to isn't fulfilling.
It can be all sorts of reasons. superficial or deep. but in the end, almost all affairs are based on fantasy and not reality.
always someone falls for the other more; or reality comes crashing into their idyllic fantasy.