Oh dear, that would be awful for me.... I don't get on a horse without anti-jiggle spandex undies...
Seriously, I wear lycra type exercise or bicycle shorts because I used to get chaffed.
I used to wear long lycra pants but they got so worn out and I haven't been able to find any more so I switched to the t-shirt material. They aren't nearly as slick in the saddle as the lycra. Plus, I looked like a watermelon in a grape skin in the lycra pants. The others are thankfully a little looser.