Ummm... I'm not sure what the hell the other posters are trying to tell you. Most of it I don't agree with, but I won't even elaborate on the advice they're giving you.
There is absolutely no reason for your girl to be locking lips with any other guy. NONE!!! If the shoe was on the other foot, how would she feel about you slipping your tongue into another girls mouth? Pretty pissed off right? You'd be a dog amongst the other names she would call you. She would get all of her friends to see her side and they would turn on you also.
The next part. The fact that she locked lips with another guy is grounds for dismisal. Then to add insult to injury, she didn't hide it, but pretty much threw it in your face. "HA!! See what I've been doing behind your back!!?" Because you're all puzzy whipped, you're not even gonna say anything about it for fear of losing her puzzy?
Overall, you might hold the title of boyfriend, but it's obvious that this girl is seeing other guys. I believe that she has already broken up with you, but you're too closed minded to realize it. The minute you confront her about it, she's gonna blow up about the whole thing. All of these bones are gonna start flying out of the closet hitting you in the head and then she's gonna slam the door by hanging up the phone leaving you wondering what the hell you did wrong.
From here, you're gonna go through a self pitty phase where you start listening to depressing r&b/country love songs that reminds you of her. You're gonna start flipping through pictures of the way things were between the two of you. You will start to hate life. You will continue to check her myspace/facebook for any sign that she might be thinking of you, only to find that the guy you saw her in the picture with is leaving her comments of how he can't wait to see her tonight, which will lead to jealousy.
Then, you'll go into "I can win her back" phase. You'll check this site on advice on how to get her back. We'll tell you to forget about the bit#ch, but you won't listen. You'll go out and buy her flowers and cute crap which she will not even want and probably won't take. You'll start calling her, but she will never answer or return your calls. You'll begin emailing her and sending her comments on myspace/facebook, which will be deleted until she finally blocks all messages from you. Then you'll turn to anger.
You'll suddenly hate this girl. Leaving her hate voicemails and messages. Spreading rumors about her. Trying to get your friends to turn against her like her friends did to you, but they actually like her because she has a cooch and figure that one day they will be the one banging her since she cheated on you.
Eventually you'll find another girl to replace her and you'll wonder to yourself why you even did all those things you did. You'll wonder why you were even with her in the first place because this girl that you're with now is so much better. She treats you so much better than the last girl. Then you'll forget about her completely. It'll be another memory in the back of your head. You'll go on through life happy. One day you'll even find out through a friend that the guy she was with cheated on her and left her high and dry. You'll feel good about that because she would have gotten what she deserved.
Last part sounds good right? Well, how about you skip all that crap in between. Dump the girl. Forget about her completely and find another girl to pick up the pieces so you can skip the b/s and get on with your life. Trust me... you deserve better. Read the bible and start spinning some more plates