Nah. It's entirely possible that Susan's intentions are benevolent here.

Being able to trust that both of you are committed is critical to making a marriage work. While it's possible to trust each other's commitment and have a prenup, in practice it's not that simple. Relationship's have a delicate equilibrium of trust that needs to be maintained— a prenup does make this more complex. I'm not saying it's impossible, I just think being hesitant about a prenup is entirely sensible position.