I'm still reading this, I just haven't sat down with my e-reader for long stretches of time in a while.
So, I knew going in that this would feature a poly relationship, but I didn't know the specifics. I still don't 100% know the specifics, but here's how things seem to be shaping up:
Nik's family is aristocratic but suffering from money problems, and they're trying to fix that by marrying Nik off to a wealthy young woman. One of their top choices happens to be Wisteria. Wisteria is already resigned to the idea that she'll end up in a loveless marriage. Bare minimum, she'd at least like it to be an honest one, and she'd like to occasionally have sex with a willing partner. The marriage contract she wrote up lays out the possibility for her and/or her husband to have lovers, as long as everybody is discreet and she and her husband are honest about it with each other.
Wisteria doesn't have a lover in mind for herself. However, Nik already has a secret lover: Lord Justin Comfrey. Their relationship has to be a secret because same sex relationships are more than frowned upon by society. Justin really wants to help Nik with his money problems, but Nik finds that idea, combined with their sexual relationship, to be distasteful.
At this point, it looks like Nik might end up marrying Wisteria because of familial pressure and because he also kind of likes her as a person. He and Wisteria will arrange things so he gets to continue his relationship with Justin. What I'm not sure about is how things are going to go with Wisteria. She's attracted to Nik, but so far he isn't attracted to her. Maybe that will change?