Yeah, I was so happy when he was finally about to stab that bitch, but then he fucks her?!
I would have, too. Sorry.
Is that also hard to believe?
Even though this will probably turn out to be a big mistake - similar to Lila -, I kinda like this development. Visually she is pretty close to Rita (which is why he fell for her on first sight) and contrary to all the things around him - who pressure & try to control him - she is pretty passive. She's pretty manipulative this way.
And don't forget, that Dexter is much less self-controlled than he admits: He missed his brothers true identity, let himself be turned on by Lila, let himself be bullshitted by Miguel and caused his wifes death, because he desperately wanted to kill Trinity himself. Dexter is extremely smart, but he has his dump spots. Which imo makes him more interesting.