I think it is important to view this in perspective. When Nick slept on the couch, he had just discovered Juliette was a hexenbiest. He had no idea what he was dealing with. Up to that point, he had only bad experiences with hexenbiests. For what Nick knew, Juliette could have tried to kill him in his sleep.
Adalind and Nick having a child made it possible for them to trust each other. So, in a way, it is probably true that he would not have slept with Adalind if it were not for Kelly. But he was attracted to her already in the Pilot and as Adalind said: “under different circumstances, I think you and I could have really had some fun”. In fact, when the circumstances changed, they did.