Robb could've married the Frey daughter and had Jeyne too. Kings always had side pieces
naw he has to much honor for that.
Also he saw the way his mother treated Jon and he prob don't want that for his b*stards. Catlyan is the main reason why the Starks are no more and the north is ruled by the boltons. She lets Jamie go, treated Jon Snow like shyt so Robb is emotionally scared that he can't even fukk a girl without marrying her. And that led to that set up.