Whether or not that would be considered "cheating" would depend on the relationship you have with your girlfriend.  But, I think in most cases, one would say that you cheated.  With that said, its probably not a big enough deal for your girl to ditch you over, if you all have a good relationship otherwise.  

Even so, I definitely do not think you should tell your girlfriend.  Nothing good can come of that.  You feel guilty because you fucked up.  Don't make that her problem.  Just take it as a lesson, and don't do it again.  

Alternatively, if you think you want to be able to do that kind of thing with less guilt, maybe you should tell your girlfriend that you'd like to loosen up the "rules" of your relationship.  While most women aren't okay with people making our with other chicks, there certainly are some people that are okay being in a less restrictive relationship.  

I'm 37, and have been married for 4 years.  I told my wife recently that if she hooks up with another guy, or "cheats" in any way, that I don't want to know about it unless she wants to end our relationship.  I've been around long enough now to know that shit happens in life.  I also know that my wife is a human, and has thoughts, desires, and needs.  I told my wife that I want her to be happy, and if she feels like fucking some other guy will do that for her, that she should go ahead and do it guilt free.  I also told her that it would hurt me to find out about it.  So, the only condition is that she has to keep it to herself.  If she feels bad about it, she can't rid herself of the guilt by telling me the hurtful truth. 

I've cheated in previous relationships, and I've been cheated on.  The biggest problems with the cheating weren't the cheating itself.  The problems come from the guilt and anger that the couple deals with later.  So, once again, at the risk of belaboring the point - just keep it to yourself, go on with life, and learn from it.  And, if its not already, make this relationship, or your next one, into something that works for both of you.  Don't worry about sticking to traditional rules.  

Agreed. After reading his edit, I can certainly say kissing is just so meh especially with drunk people in bars. Imagine he goes to a strip club instead, much worse but more acceptable to people. I also wouldn't want to know about it months later which leads to...

Agreed. I certainly wouldn't want to know, especially something so minor as kissing and after so many months. Although I would definitely take a wife much more seriously then a GF. It would just change my opinion of the other person for the worse. Sometimes skeletons in the closet come up 20 years after the fact and couples fight over something that is just so long past, it is just so pointless.  The relationship has bigger problems if they can't leave something in the grave like it should be.

So have you talked about boundaries or anything like that? Telling that you were drunk and a girl kissed you, but nothing else happened could be a good way to talk about what is accepted and what is not. You could ask if that was okay and say that you are sorry if you crossed the line by letting the girl kiss you. She could also benefit from it as it might not be that beneficial to be overly jealous or possessive. If you really like your girlfriend then telling the truth is probably the best option, depends on the person of course, but think what she would want.

There are problems in every rellationships and how they are dealt is very important. Avoidance is not often useful, learning to fight and argue in the right way and learning to forgive and say sorry are far more important. Whether she can or cannot forgive is her choice and telling or keeping it a secret is yours. I recommend telling for you and forgiving for her.

Deep down you seem to already know that you need to tell her, so do it and don´t let fear stop you.

As always though, you run the risk of the person finding out via other means, and it's better to pre-empt that by telling them yourself.

If you do tell, it is better to say it day 1, I agree. But since its been months, I think it would be better to not tell. Would just cause more problems, and if found out, he could probably say he doesn't remember the exact details of that night. But since she is possessive and jealous type, it is going to be bad no matter what. (which is why I don't look for that type :P)

You should tell her.

I probably wouldn't, but its thw right thing to do.

I know somebody who was a best man at his friend's wedding, while at the same time slept with his friend's wife.

If I were you, I’d tell, had the same situation happen a few years back. I felt really guilty after holding it in for a week. Luckily she forgave me. All in all it’s better she hear it from you instead of someone else anyway.

Edit: I removed my previous post as I think you want a serious discussion.

I would own up, explain that you were out of your depths and you were drunk / whatever and apologise your ass off, you will feel better for doing it, even if she chooses to end the relationship over it then you'll be better off that way and she'll know that you are more trustworthy in that you were honest about your mistake.

You'll be eaten alive by the guilt if you don't come out and say it to her, even if your relationship continues for life... it will always be in the back of your mind, afraid that the girl will say it... afraid if you are ever drunk with your gf that you'll say it by accident, you wont be able to really ever relax with it on your mind.

Honesty is all you got really , if that ends the relationship, move on, there is plenty more fish in the sea.