If you were about to go skydiving and you knew there was a 60% chance your parachute wouldn't open, would you go? Would you at least make sure you had a backup parachute? You're in love and you are going to get married, despite the odds not being in your favor. The prenup is your backup parachute.
What if you marry someone who ends up refusing to work? You're married for 10, 20 years, you bring in all the income, you've worked your way up the employment ladder to get to where you are, and your spouse refuses to contribute. Since Colorado is a "no fault" state, you may end up paying maintenance (alimony) for a number of years, because the Court will not take into consideration the spouse's refusal to work. It's just about the numbers. A prenup doesn't just protect what you already have - it can protect what you will have, and protect you from unexpected circumstances.
If you're afraid to approach your fiancee with this idea, consider this: A well-written prenup is not one-sided, contrary to popular belief. A prenup should be reviewed by two attorneys - one for each of you, and should protect both parties equally. It's a contract, and you can put whatever you want in it to make sure both parties believe it's fair. You can honestly tell your fiancee that the prenup is for both of you.