Drunkenly kissed my friend who has a girlfriend. Any advice on how to handle this?

My friend and I had been out drinking and decided to head back to mine to chill out. I usually do this kind of thing with friends and so it wasn't really out of the ordinary for us to be alone. We had been drinking loads and decided to play a game of Truth or Dare using an android app. It started off as a total laugh and we skipped the dares when they got inappropriate and so it was fine. However, eventually a dare came up saying to kiss each other and we actually kissed. Problem is that he has a girlfriend and so I really don't know how to handle the situation.

I talked to him on Facebook today and neither of us mentioned it but I have to see him tomorrow and so any advice on what to do would be great.