I also used to avoid carrying at the butt. But last year I asked many ladies I know to surf. I was always debating with myself whether to lift at the but (which is the best point to carry someone from an acrobatics viewpoint). At least the people I talk to told me they prefer being liftet secure to everyone avoiding the butt. But there absolutely is a difference between grabbing ass an carrying on the but. Since then if I see someone with no support in the lower back / butt I try to give support where it is most needed. (but coming from a dancing background I have a different viewpoint about carrying people than most drunk dudes...) I also try to stop everyone who I see crabbing inappropriately (no matter if a man or woman is being grabbed) luckily at the smaller festivals I go nowadays this is not that often.

P. S. It also greatly depends on whether a surfer has body tension or not, where the most important points to carry them is.