Dude, first of all, don't listen to what people are going to say. Cheating isn't for the "scummiest of scumbags" like that one mod said. Don't get me wrong, it's not right. It creates lots of problems and hurt feelings. But are you the lowest form of scum? No. You're human, made a mistake, and as long as this type of behavior is out-of-character, then you're not a terrible person.
People who judge are usually just seeing things at face value. Virgins like to toss out judgments because they don't have a clue. Trust me, women can be HUGE whores. If they judge you, just remind them of all the fucking around they did with another girl's man. I just hope that if you are prepared to stay with your girlfriend, (if she forgives you) you get used to the fact that all her friends and family aren't going to like you very much. They are also going to take this one out-of-character moment you had and make it your "true colors". Just realize this going in. Each and everyone of them are either hypocrites, or have just never been in a situation where they COULD cheat because nobody wants to fuck them.
And let's say you find people who have never cheated, and they could have, but said no... If they start talking trash, just remind them that they don't have a clue, that they are being ignorant. It was happenstance that they ended up being the type of person who never cheated. Pure blind luck. It could have gone another way. It's a 50/50 shot, AND they haven't finished living their lives. They could become a cheater too.
Anyway. My advice? Come clean. If your girlfriend leaves you, whatever. It's not like it matters. You can obviously get pussy. lol Okay, seriously.. If she forgives you, it's actually going to be more Hell than if she doesn't. You might break up and go through some depression, but that is not a big deal compared to if she forgives you. If she doesn't that, then she's not really forgiving you. She won't forget it. She'll bring that shit up in every fight. She'll tell everyone she knows or who will listen what a jerk you are, and then make plans for all of you to hang out. Putting you in the same room with a bunch of people who hate you. And this will be often. This will be every single family and friends get together forever.
You know what? Lie. Hide it. Deny everything. If you want to keep your girlfriend, you can't be honest. It would kill some part of her, and blah blah blah...
In fact. You know what? Fuck it. Just push that other bitch down the stairs.