I think smackie9 is dead on. It's easy for the person who has fewer assets and who makes less money to say, "Well, I gave a lot to the relationship, too, so I deserve at least half," even when that person didn't make nearly the same amount of money as his/her partner did and didn't have nearly as much to show for a lifetime of work.
It's very simple: A prenup protects everyone. OP, if he comes into the relationship with significantly more assets than you do, he gets to keep those assets if you divorce. If he makes, for example, three-quarters of the money in the relationship, then he deserves three-quarters of the money if you divorce.
Marriage *is* a legal contract. It should be treated as such. If you have any problem with signing a prenup that gives back to each individual party what he or she earned over the course of the relationship, my bet is you don't really want a mutual relationship; you want someone to completely take care of you financially.