Really? You're going to Moab to mountain bike, you want to have a nice dinner... you can't take a shower and wear clean pants? What are you doing, sleeping in the back of a car? That seems a bit extreme, especially after two or three days.
Trouble is, most any long pant is apt to be pretty warm if temps are above 60. That translates into sweat, which translates into damp pants, which attracts grit and dust. Not to mention chain grease.
I supposed you could try one of several makes of travel/hiking pants. They aren't really bike-specific, but you always can wear padded undershorts if you like some padding. Prana's Stretch Zion pants ($70) are made with a nylon/Spandex blend that has some stretch, breathe well, and look good with a casual top. The charcoal color will hide dirt better than khaki. I also like Arc'Teryx's Raider Pats ($119), which have a nice urban-warrior-chic vibe to them. They have a technical cotton/nylon blend fabric, and articulated knees for easy movement.
You might opt to get some mountain-bike shorts, and if it's cool, wear some light tights under them. Dinner time, the tights come off. Zoic's Men's Tourist shorts ($69) are just the ticket, a bike-friendly pair of shorts made from a microfiber fabric that is durable, dries fast, and looks good even after a day of abuse. Cannondale's Quick Baggy Bike Shorts ($60) offer similar advantages.