I tend to think the primary reason Edmund puts up with Mary's insults, and tries to put a positive spin on her faults, has much to do with the fact that she's an attractive, charming young woman for whom he has feelings, and who he believes has feelings for him. In other words, like many people in love, he sees what he wants to see and hears what he wants to hear, and makes excuses for what he can't completely ignore. His hormones are over-riding his brain.