Holocaust involves mass murder....lots of suffering & pain.
Slavery involves forced servitude....lots of suffering & pain.
Marriage is two or more people entering a contract out of love or binding clans together....(depending on circumstances) no suffering & pain.
Rape involves a violent assault with sex as a weapon...lots of suffering & pain.
Pedo/Hebo/Ephebophilia involve messing with kids, who can't give informed, reasonable consent (it gets a bit murky with Ephebophilia involving much older teeners, who are usually well-developed but even then, they're still immature kids with brains that haven't fully matured, even if said teen was a gorgeous young hunk or voluptuous babe...they're STILL KIDS)....more suffering & pain (let the teens sex-up each other, adults are off-limits).
Zoophilia...It's the rape of an animal, which can't consent! It's animal abuse, like neglect, beatings, torture, dogfights, cockfights, etc....suffering & pain for Bowser, Fluffy, Bessie, Bonzo, Poly, Seabiscuit, Lyle, Baloo, etc.
Look, if there is one moral truth that is constant & written in stone, it's this....IF IT MALICIOUSLY CAUSES DESTRUCTION, SUFFERING & PAIN AND/OR INVOLVES ANY FORM OF PREJUDICE, IT'S WWRRRRRROOOOONNNGGG!
Otherwise, morality is fluid & everything's a shade of grey, thank you.