Also, Marten's one to constantly second-guess his relationships with women, and whether they're attracted to him, to the point of coming up with the Second Law of Sexual Dynamics. Plus, he had several rationalizations that let him put any relationship with Claire aside.
But, the tension was always there, and had been there since at least the wedding on Marten's part, and probably the lake party on Claire's part. Add in some alcohol, and Marten begins to realize that he likes spending time with Claire, and finds it more natural than he's ever felt before, because they trust each other and feel comfortable in each other's presence - which, to be honest, is probably something that Marten's never quite had.
Vicky felt the need to bail across the country from him.
The entire gag for the first 500 strips of the comic was about how Faye was physically abusive to him, and didn't trust him with her history.
Dora didn't give him a chance to recover from that, and shocked him into a relationship with her. And, while he genuinely did like her... she didn't trust him, ultimately.
Padma... he was never comfortable with Padma, and there was an expiration date on that relationship.
Delilah, well, that was a one night stand, so...
Claire? She trusts him, he trusts her, they're comfortable around each other, and in a moment of vulnerability for both of them (Marten being drunk, Claire having been forced to think about it by Faye), they connect. Claire realizes that she's been attracted to Marten all along, Marten realizes that he's liked Claire all along, and then the next morning...