I had that SAME fear. I'm really sensitive and I hate wearing shorts that don't have a lot of padding.

About a month before my race I started wearing my tri shorts on some 20-30 mile rides. My butt was fine. I think it was broken in from 400 miles a month on the bike.

So I wore tri shorts in the race and my butt didn't hurt AT ALL. You would be surprised.

Good luck!