Regardless of WHY Carmela chose to sleep with him, it wasn't too smart of him to go along with it.

Remember the old Jim Croce song -
"And they say,
You don't tug on Superman's cape,
You don't spit into the wind,
You don't pull the mask of the ol' Lone Ranger,
And you don't mess around with Jim".

Add to that, "You don't sleep with Tony Soprano's wife".