Absolutely do a prenup / cohabitation agreement (unless you each have no assets/pension). You need to protect what you (and your spouse) worked hard to build for you and your child(ren). This chapter 2 may be a perfect relationship and may last a lifetime but there are so many failed relationships out there that started out wonderful. When it gets ugly - it gets ugly.
My chapter 2 guy is in the process of moving in and I have spoken to my lawyer. I want to protect myself and my son but I also want him to feel that he and his girls are also protected. Neither of us are together for material things (or lack thereof!) so signing a piece of paper to make sure we are protected in the future - just keeps us safe.
Should we choose to get married some day then the cohabitation agreement turns into a prenup. I will need to adjust my will at that point to reflect any changes I feel need to be made. I have no intention of using it as a "get of of jail free card" I still believe in the working things out and doing what you can to make things work. However, sometimes there comes a point, when there is no point of return.
Just my opinion.