how many different ways are there to describe a sexual act?
One of the reasons BDSM stories tend to be longer than other stroke stories is that there are so many ways to tie the girl up that leave orifices available. And A, P, M are not the only ones when sadism and masochism are involved. Eyes, ears, nostrils, throat (don't need to go through the mouth for that one, if you have a knife or scalpel), navel (sailors like naval maneuvers), urethra, cervix, armpits (hey, a pit is a hole), the holes in the nipples (human cows or girls with pierced nipples), holes in the clit (more piercing), toes and fingers, or between them and there may be more (freckles, hair follicles, skin pores) we don't have to require large openings to manufacture holes. Needles, skewers, screws, dildos, baseball bats and beads, lots of rope and chain, the methods and parts used in BDSM are if not infinite, much larger than just plain "much sex." Not to mention a wide variety of whips, canes, paddles, and other striking examples. There is so much to BDSM they use D and S twice. (Discipline and Domination for D, Submission and Sadism for S.)
Now it is a squick for lots of people, particularly if you do it right.