The most honest and appropriate thing to do? Tell her.
Most males on this post have said don't tell her- girls find out absolutely everything one way or another, and I promise you that. The guilt will overcome you. If you hide something now, you'll have negative thoughts and may not be truthful in the future. Yes you may argue and whatnot, but if there's a chance that she'll forgive you then isn't it worth it? But to be perfectly honest, and being blunt is what's needed... Are you sure you're in love with her when in your drunken state you'll kiss someone else?