If you have significant assets going into the marriage, then a pre-nup is a good thing. If not, then it probably doesn't matter. You can preach this over-romanticized notion of marriage being forever and blah blah blah, but the harsh undeniable reality is that a large percentages of marriages fail for one reason or another, and you should make efforts in advance to prepare for that situation. If you had a 38% chance that you were going to get into a car accident, you'd probably make especially certain you're protected as best as possible, no?
If talking about a pre-nup and the serious subject of assets prior to a marriage introduces doubt or mistrust in the relationship, then it isn't that strong in the first place. Marriage is a contract too, just like a pre-nup.
My opinion is that if you decide to formalize your relationship with the government and/or religious group, and that relationship fails for some reason, then everything earned or purchased since the day you were married is split, but assets held individually prior to the marriage should remain in the possession of that person after the marriage. This would seem like common sense to most, but we have to state that in contract unfortunately. If a contract that really states the obvious scares away your bride, then you can go back to doing what you want, when you want, however you want, with whoever you want, and spend as much as you want. bummer.