No, Marten's not solely motivated by sex, otherwise he would have dropped Faye completely as a friend after he and Dora became a couple. He'd just be saying "hi/bye" to her as he walked past her in the apartment, and not much more than "hi, is Dora here?" when he came to the coffee shop. He also might have kicked Faye out of the apartment after "The Talk" if all he wanted from her was sex.
I'm just saying it's only natural that when people get into romantic relationships, they tend to spend more time with their lovers than their friends, at least early on in the relationship, which it still is for Marten and Dora. I tried to say that in a joking manner in my post above, guess I shouldn't have left out the smiley. In fact, maybe I should edit it and add a couple...