>>212336332
This.
He knew his relationship with his wife was built on a lie and he knew absent the lie she would never have stayed with him (this is proven true in the later seasons when Betty realizes what's going on).
Every night he slept with his wife felt like sleeping next to an atomic bomb that could go off at any time; one he found annoying and superificial which is why he stepped out of his marriage so much.
None of this "excuses" his behavior to be clear; he made the choices he made and no one was responsible for how shit ended up besides himself.
But ultimately to OP's question its not surprising at all he decided to fuck some random Jewish department store owner's daughter just like it isn't surprising he fucked the art hoe in queens or the comedian's wife. ln every case he felt like he could be more honest and relaxed around complete strangers then he could around his wife because the truth was he COULD.
That was the life he had built for himself. That was the sort of marriage he had.