Sven's story: historically casual/shallow man, used to relationships that started with sex and often didn't go anywhere else, decides that he has met a woman who's totally worth a change in his behavior. Her disapproval of his womanizing ways challenges him to maybe get a little more serious about sex and relationships, so he tries. He approaches her with an offer for a relationship that doesn't start with sex, and is rejected, because she does not trust him and she believes that he is being dishonest, and will in fact only care about the relationship for the sexual aspect. She appears, then, to be turned on enough by the prospect of sex with him that SHE starts a relationship with sex, turning the tables on him rather neatly and giving him a bit of a headfuck. He rapidly recovers and falls into the pattern of casual sex that he used to be in, going with the flow that she offered when she first started a sexual relationship with him. He is now, presumably, willing to engage in a casual relationship with her (within reason), and also possibly willing to engage in something more if she seems receptive to it (which so far she has not).
Faye's story: historically neurotic woman, who finds it very difficult to trust others enough to be intimate with them, believes that her friend's brother is precisely the type of man who is to blame for people feeling the way she does (in that he seems to be cavalier in his treatment of the intimacy of others, and does not seem to be capable, or at least is unwilling, to engage in deeply intimate relationships). This, of course, repulses her during a time when she is desperately trying to come to terms with the blockages against intimacy that she carried around inside herself. After a series of situations that teach her that those blockages are doing far more to hurt her than anyone outside of herself, she decides to try to break them down by throwing herself into a sexual relationship with someone who seems to be safe (in that he will not ask her to be intimate with him). When he acts like someone who WOULD in fact like intimacy, she responds with skepticism and rejection, beliving him to be using the same old womanizing tricks on her that he must have used on other girls (in reality, the only "trick" Sven seems to use is the trick of telling a girl that he is attracted to her and hoping that the feeling is mutual). However, the intimacy-building influence of having sex with someone is steadily wearing down on her emotional blockages, bringing her out of her shell.
This is a great situation for both of them. Faye is challenging Sven to improve himself by getting better at deep intimacy, and Sven is challenging Faye to improve herself by overcoming her aversion to intimacy. They're good for each other right now, as long as they don't make any huge, dumb mistakes.