Some folk in the sex and relationship advice field think its ok to go off and look at porn, or flirt with someone, or think about what having an affair with someone would be like when you're in a relationship. That its not cheating. I don't think its ok. If you're in a relationship, then you're in it and should take care of it and focus solely on your partner. If the person you're with isn't 'enough' for you, then you shouldn't be sticking around but getting your fix somewhere else. Find someone who is enough, and let the person you're with do the same. I'm going to write a list of what cheating means to me, and I'd love to hear what you think... There's no right or wrong answer to this, so long as you're happy with your relationship. If you honestly don't mind your partner checking out other women etc, and you are letting him do it for your own healthy reasons, then there's no problem. But if you are putting up with something from your man which hurts you and leaves you feeling cheated on; because you are afraid of losing him, because you think you're not good enough, or because he won't change for you, then please please realise that a) none of this is your fault, b) you deserve to feel safe, loved and respected in your relationship, c) if your someone is not willing to cut out an activity that hurts you then he is probably not a safe bet: if you love someone and/or respect them then you do not want to see them hurt. You should mean more than Playboy.
Ok, so here's MY list, which is just that, my list... this is the standard to which my man and I hold ourselves. Every relationship is different. Just remember that you deserve to be treated the way you want to. Please write your own lists, if not here then in private... and give your own feelings a good look over anbd see if your needs for a relationship are being met.
Right.... list:
Pornography
Fantasy involving other people
Checking out other people
Flirting with another person
Thinking about a sexual or an emotional affair with another person
Kissing someone else
Touching someone else in a sexual manner
Having sexual feelings/thoughts about another
Sex with another
Might sound a bit strict, but to be honest, its not that hard to be monogomus with the right person. The thing is finding the right person, and weighing what you get from actions like those above against what you get from your partner.
I hope I have made some kind of sense and haven't made a blithering idiot of myself... lol Anyway, drop me a line or a list, or even a hi....
ajnspencer
Pro 
I think you've hit it on the head when you say it's about the happiness of both parties.
I know couples who have jokey "allow" lists of celebs they could sleep with if the situation came up and both are happy with it, I've been there myself.
Look around this blog and many a married person will post photos "look how gorgeous this bloke/woman is" and mostly it's done in fun with both parties in on the joke. (Porn comes under this too, some couples like sitting down together and watching something raunchy.)
Attraction is a physical thing and even in a loving relationship is almost impossible to wipe out. Love however is emotional and is much more important.
I've had the conversations "Oh wow, I'd do Brad Pitt... Really? Great, you get him out the way I'll have a quickie with Angelina..." Cue laughter.
When it's a problem is when one party is left out or feels betrayed by it, then even things in your head become cheating.
As for the rest of it... //anything// physical is cheating in my mind, it's differing levels of cheating but it still is.