Originally Posted by PaintHorseMares
Exactly. If you can ride a bicycle with those tiny, skinny seats without hurting yourself, you can ride a horse with no problem at all.
As a very experienced bike rider, it's quite a different thing. First, if you are road biking, the bike is not bouncing, nor is anything else. (In fact, the problem on long rides has more to do with steady pressure.) If you're mountain biking on terrain rough enough to bounce, you're standing in the pedals, so your crotch is not contacting the seat.
As a not-very-experienced horse rider, the problem I have comes from bouncing, and is only at the trot. At a walk or canter, I can sit in the saddle without bouncing, and have no problems. Sometimes I can manage to sit a trot without bouncing, and again no problems. Or I can do a standing trot that's high enough so that the low point of me is always higher than the high point of the saddle...
And yes, jockey underwear. I can't imagine trying to ride in boxers (not that I would ever voluntarily wear them).
PS: You know, I just realized something. If I don't think about it, I tend to ride leaning forward too much. I'm unconsciously trying to take a proper biking position...