The difference being that sexuality is typically considered an innate desire, whilst the desire to commit violence is not.
Plus, we as a species have a biological imperative to protect our offspring, but apparently an immense capacity to ignore violence committed against others.
Monkey see. Monkey do. Nothing surprising here. Once people decide to do something then they model their actions on what they've seen. Even for such innate and strong desires. So completely hands off approach, leaving it to market forces, might not be the best course of action. But banning doesn't seem like a golden bullet either.
Plus, we as a species have a biological imperative to protect our offspring, but apparently an immense capacity to ignore violence committed against others.