Darken12 said:
It's about empathy, really. There have been studies where people were asked to rate how excusable it would be if they cheated on their partner, and then how excusable it would be if their partner cheated on them. Most people were quick to excuse themselves and to declare certain activities (kissing, some sexual acts, emotional cheating) as "not really counting", while simultaneously being very unforgiving of others cheating on them.
If you can empathise with your partner, you know what's so bad about cheating.
Now, open relationships are completely different and are in no way related to cheating.
The trick is to learn through experience, if you know how much it sucks to think you're special to that person and then find out you're one in 2 or more then you're far less likely to try and excuse yourself from it.
Having open relationships is perfectly acceptable so long as everybody knows and agrees that it's fine, sneaking around behind your partners back about the whole thing is not however, because as somebody said it betrays their trust and trust is a very important thing for relationships.
Interestingly the whole monogamy thing is part of why I'm terrible at getting boyfriends.
Edit: In short, don't maintain or start relationships if you plan on sleeping around, keep things casual, you'll cause less people less misery. The horrible feelings of betrayal and inadequacy really do have deeper effects than people like to realise.
Edit edit: Oh, and don't cheat on crazy. So someone like me? Yeah don't do that, it's how you end up with a beehive under the floorboards or something. Or a skunk in your wardrobe. Or pictures of you on the internet.