I wish I could wax as eloquently as some of the rest of you have, but to me this is a very simple matter: There's nothing wrong with a poly- relationship, and Tourguide provides an excellant explanation of polys, BUT there is a huge difference between polys and cheating.

If you have your partner's permission, you aren't cheating. If you don't have your partner's permission, you're a bastard (even if you tell yourself you're doing it to make yourself a better spouse).

In a different thread I read through tonight, someone was essentially asking whether it was all right to cheat on a cheater. The very wise answer she was given (by multiple posters) was, NO. Even if your partner cheats, you should break it off with them before you take up (or have a fling) with someone else.

And that has nothing to do with religion -- it has to do with common decency.