Yes - cheating can go this way when partners have never discussed options or therapy, and perhaps they don't have a kid together. Also, yes - you can be an ass and expense your partner to illnesses. Equally valid can be the case one partner has addressed the issue, proposed therapy, has been rejected, but still, provided there are kids involved, decides to stay. As long as they protect themselves and the other, cheating can help the partner who suffers from (insert root cause for cheating) to cope with it. As the saying goes: what you don't know, doesn't hurt you (and yes, - when you get STD, you know).