Now Marten will try to understand what has happened in the most melodramatic way possible; however, there is no understanding Emily. You just go with it!
The big question is if it is just a smooch, or if Emily is also grabbing Marten's ass with her left hand. Obviously, Marten has never considered a romantic relationship with Emily, and is completely taken by surprise. Knowing Emily, he might just consider it part of her usual weird ways without any deeper significance; but he might also start wondering if Emily really has a crush on him, and how he should respond.
Emily has shown affection towards Marten
before, but nobody thought it was significant because she was drunk. The
"love letter" made Marten momentarily confused, but it was quickly resolved (perhaps the others that were invited did not get pink hearts on the envelope?).
Emily's only friends are through her library internship, as evident
here. Marten acts as a magnet for attracting girls with problems (Faye, Dora, Hanners, Marigold, Cosette, Emily, Claire), getting them friends and a social circle.
Tai and
Faye have earlier discussed the possibility of Marten hooking up with one of the interns, where Marten dismissed the idea. He did not seem to have an immediate crush on either of the interns at the time. However, he was a bit intimate with
Claire afterwards, and now Emily, but he did not take the iniative on either occasion (on the other hand, whenever does Marten take the initiative?).
My guess is that Marten is not really interested in a romantic relationship with Emily, and will try to handle it the same way he did with Claire. He will just try to act as normal towards Emily, and hope Emily is not making further advances. He is going to worry a lot about having to talk to Emily about his feelings for her.