in general the blame for cheating seems like it should be assigned to the cheater, not the person he/she is cheating with.

Yes, it'd be my mother I'd mainly be pissed off at; but if Bob was aware she was married (and in that hypothetical he definitely is aware she's my mother -- though he might have found that out later)...

... but this thread is probably fast approaching an entirely too tangential state relative to the main post.

Agreed.