His girlfriend never discusses any predilection for kissing girls, drunk or otherwise.

Then, she gets drunk and does it one night, months into a relationship.

Either:
a) she's always had that predilection, laying dormant, never acting on it, and she could have discussed it with him. Which would be, in the grand scheme, uncool.
b) she did it for the attention, in which case, really fucking uncool on multiple levels.
c) she needed something fulfilled that night and she didn't go to her BF about it, going to a stranger instead. Which, again, really fucking uncool.

Of *course* dude doesn't know how to feel. His girlfriend just did something really fucking uncool.