Juliette is one thing. She and Nick picked each other out and we don't know the basics of how they formed a couple. But I think Nick limiting himself to Adalind just because she has his baby a very odd aspect to the series.
It's not a rare occurrence for a man to stay with a woman for the sake of family/their child. Many men are trapped in marriage for the sake of their children and it's not rare for women as well to drop the baby thing for men to stay with them.
And regarding someone like Nick, he's a person who's very responsible with high regards to discipline and **** like that. As we can see from the series, he's a family guy who has a history of a broken or well almost-absent family. If we look at it on a psychological point of view, he has two ways: either stay together with the woman who's the mother of his child or end up broken like his family.
And broken family does not pertain to just divorce and all that, I'm not sure for the term to use there (so please excuse my use of the term broken family) but, the possibility of him thinking he would not be able to protect his family if they were not together is high. So he has a lot to lose if he does not stay with Adalind.
In all honesty, I think Nick and Adalind could love each other. But, with Juliette still alive and in the picture? We all can't deny the fact that if Nick was given the chance he would still choose her. If it wasn't for all the other factors of course: a.k.a. his son and the family he always wanted (and prob really wanted to have with Juliette if given the choice)