If all that happened is that he kissed another guy and if you do decide to let it go, then you really need to let what happened go. Don't use one small mistake against him again, that will only harm the relationship further. Before letting it go, you do need to tell him how much it hurt you that he did that. And to please not let something like that happen again because you will not be as forgiving if there is a next time. But once that is said, don't bring it up again.
Agreed. I wouldn't really blame him for, as a number have mentioned, "putting himself in a situation where this could happen." You said he just finished his master's degree, right? And was at a graduation party? I think a party is totally appropriate after accomplishing such a feat. And I'd definitely let my man fly solo if I didn't feel the crowd, didn't know anyone, or wasn't in the mood. I'd celebrate with him mono y mono if I wouldn't make it, but I certainly go just to be a killjoy if that's the situation I was putting myself in.
At any rate, I would be very clear and forthright with him--which, admittedly, is easier said than done. This is something I struggle with, of course.

Nonetheless, I think it's paramount in a long term relationship to eventually get over the discomfort and reservation. Some of the hardest things that need to be said are the most important things to be said. I've seen this with the president of the company I work for, and I have been learning it in my interactions with subordinates on whom I have been too lenient .
I would wait to cool down a bit, but not let the infraction slip by, or let too much time pass by, which can detract from the severity of the situation. And I would definitely be honest about:
--feeling hurt that he DID (as opposed to "would," because then you're calling him a sleaze--stick to the facts) kiss another man
--the newfound anxiety that this could possibly mean something more
--extensive appreciation for his honesty
On the last one, you might even note that his honesty is what has brought you, calmly, to a point of communication--as opposed to ripping his head off or showing him the door.
I suspect, if you don't say your piece, or get the answers you need to be at peace with him, then you will be left with a pang or tinge of regret. The decision is entirely yours, but I would suggest that the discomfort of investigation is actually much lighter than the risk of regret.
Communicate! Don't water down or minimize your reaction, but don't overreact either!
Food for thought.