When is it cheating?

**Note: I freely admit to being tipsy if not drunk while writing this.**

So, when is it cheating? I was the kind of girl who couldn't commit to anyone in college, and slept with whatever guy was cute and willing, while seeing other guys. I never realized how damaging it was to said other guys, but it was. But now that I'm in a committed relationship (aka I'm married), what is considered cheating? Is it thinking about someone else? Is it making out with someone else? Is it getting off in front of someone else? Is it sleeping with someone else?
Obviously, most people say it's getting physical with someone other than your significant other. But is that where you draw the line? I mean, in college I had two-night stands. Not one-night stands - the guy and I would usually get tired of each other after two random nights, and that was cool with me. They weren't a mystery to me anymore, and I'm sure they felt the same way. But now that I'm married, I need to actually be faithful to one person, and what does that mean?
Does that mean I can't look at other guys and admire their physique?
I don't think so. I'm not dead.
But, hypothetically, does it mean missing out on the chance to make out with another (married or not) person with whom you've wanted to make out with for X number of years? Hypothetically, people, don't flood this entry with "shame on you" comments. But seriously, does getting married mean that you've given up the chance to ever get it on with anyone else?
I think it does, unfortunately. And I know some people will be annoyed that I'm asking this. And that's ok with me. I'm used to going against the grain :P
But what's difficult is giving up the chance to experience new guys. Everyone is new. And I enjoyed that in college, no doubt. The number that I slept with was in the double digits, which is ok with me, because it was FUN. Different. New. Etc etc. I always wanted to live my life in such a way so that, on my deathbed, I would lamant what I DID do (which I swore I'd never do, and I don't till this day), rather than what I DIDN'T do. I never wanted to be one of those people who wondered "what would have happened?". I know it's pretty much inescapable, but I can do my best to stop it. :)
So, again, if you get the chance to make out with someone who you've wanted to for X number of years, and you make out with them, but you stop short of having them get you off or them watching you get off, is that good?
Is it right?
Obviously the answer is that you should never get in that situation. But why does being bad always feel so good?