If sin leads to suffering?

Attachments lead to suffering, not sin. If you're attached to the act, that would cause suffering/disenchantment.... the act itself is morally neutral. It's the way that you use it/view it that can cause problems. And those problems aren't sin and hell fire... I'm sure you can think of examples where masturbation caused issues... ;)

Because masturbation is natural and 'sin' is for weirdos.