Is it normal to sniff women's underwear?
Depends.It depends
Being turned on by the scent of underwear isn’t unusual. Whether sniffing it is okay depends on whose underwear it is and whether you have permission.
Some people find the scent of underwear arousing, and having that preference doesn’t make you bad or weird. People vary a lot in what they find sexy, and private fantasies are one thing.
The important part is consent: sniffing your own or a partner’s underwear with their okay is different from taking or smelling someone else’s without permission. That crosses a privacy boundary, even if you mean no harm.