@strawberrypomme Thank you for the clarification. It really helps put the situation into perspective. In much younger days, I’ve found myself in a similar situation, be it in the role of the girlfriend, the person with two interested parties, or the third wheel.
In your situation, what this guy did was a breach of code, if he is in a monogamous relationship. It is up to the guy to decide between either you or the girlfriend. Then it is up whichever girl he is willing to stay with to decide whether they want to continue (in her case) or start (in your case) a relationship with this guy.
I if were in either of your shoes, the answer would be ‘no’. As the girlfriend of a guy who did this, it isn’t about the fact that there was a kiss involved, but that he had feelings for someone else. It meant that he didn’t feel like this was the ideal relationship. As the ‘other woman’ with feelings for this guy, he needs to break off his current relationship before attempting to act on a new one. Otherwise, there might always be that niggling thought that he might do the same thing again. I have witnessed this type of behavior one too many times, and it is not limited to men.
Please reread @Pandora‘s post, as she provides the key. This isn’t just a case of a drunken kiss that shouldn’t have happened and can be swept under the rug and forgotten. From your descriptions, there is a good chance that this is only going to fester. Ideally, the situation should be discussed between all of you, but I doubt that is likely to happen. The bottom line is that someone needs to move out. Personally, I’m rooting for the guy to slink away with his tail between his legs.