We're always attracted to the ones we can't seem to have, I think its the challenge or trying to obtain the unobtainable etc.
Fortunately I can say I haven't been tempted by a married man, but I also don't believe you have to be catholic, or very religious to know that there's something wrong. If they were really that unhappy, why are they still in it?
Masochism? Perhaps they're into that kinda think =p

Thats how I sorta think of it =)