OK, this is pretty embarrassing, but anyone who has posted questions about the fit of shorts needs to hear this.

I'm new to biking so I've been tailing along with some experienced friends who have been showing me the road rules. One of my male friends gave me a pair of Pearl Izumi Shorts (size S). He originally bought them for his daughter to try to entice her to ride, but she never did. So he gave them to me, with tags and all.

I'm kinda on the small size. I wear size 0 or 2 in pants and thanks to my Asian ancestors, have no a** to speak of. When I put on the shorts, the lycra shrunk enough so they stayed put. Not snug, but stable. The chamois seemed a little big. It didn't wrap around my butt like my other shorts. The back end of the chamois stuck out a bit, looking as if I had a small tail under my shorts. Yeah, it looked funny, but the padding protected my girly bits and my other shorts were still damp from a wash. So I wore them.

Move forward to the next scene. We're stopped at a light at a major intersection and my two male buddies are in front of me. I'm straddling the top bar with one foot on the ground, ready to go. The light turns green. I hop onto the saddle. At least I tried.
The front, pointy end of the saddle somehow hooked onto the back of my shorts, right above when the chamois departed from my hiney. As I swung, up, the tip of the saddle held the chamois (and my shorts) down. I went up, my shorts did not. AARRGGGHHHH.

A few seconds later, my buddies looked back to see why I wasn't following. All they saw was me back at the light, "adjusting" my shorts. I didn't tell them what happened, in fear that if they laughed too hard they would drive into oncoming traffic.

Moral of the story? Get shorts that fit. You won't regret it.