Why are married men obsessed with their mistress?

Because he wants his "cake and icecream" too

B:

There are some married men that will not leave their affair partner, some of their reason will be; this other woman is so different than his wife. He already fell in love with her and afraid to loose her. His affair partner is more understanding than his wife. They will also say that she knows how to make time for him. his wife never had time because she is to busy with the children.

They will say that intimacy or sex hardly happen, they will also say that their wife always nagging and complaining. There's a lot more reason why some married men will never let go their affair partner.