The obsession with what the OW looks like--it's just like not being able to fathom how someone could be so depressed that he (or she) could lose his interest in sex--you either get it at an emotional/gut level, or else no amount of explaining could ever make you understand.
In my H's case, his OW looked incredibly like a female version of himself. I believe Jung talks about how the MLCer is coming to terms with his "anima," or female side, but ... my H looks great as a guy, but the whole she-looks-like-his-sister bit weirded me out somewhat.
