I'm not a parent and I dare say I am a little younger than you are. 24. It wasn't that long ago that I was 13 and I can clearly remember that lots of kids were kissing by that age in the playground etc. I'm not saying you should approve, I'm just saying that in my opinion it is a pretty common event.
Now if you don't want her too then it is your place as a parent to ask her not to. Policing this will be a different story but you certainly have every reason to disapprove.
I like what kirriky said. Perhaps you can use this situation to educate your daughter about sex etc. She probably is further advanced than you realise in her knowledge of this so don't be afraid that you are opening up a can of worms.
But I think it is important that you get through to her that she should be honest with you and that you won't tolerate her lying. That is a totally different disciplinary issue than the kissing.
For the record I was kissing girls at 13 / 14 and I think I'm okay.