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!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"