When the person in question wants or is having a sexual relationship with a chair. Some examples may include pounding the chair leg into your ass or vagina, another example is drilling a hole into the seat of the chair and fucking it with your penis. There are many other ways too but these are the classic ones.
Sexual fetishism is normal, as long as:
1. You don't break other's rights (such as right for not discriminated, sexual consent, and such)
2. It didn't harm everyone
3. It never done in public
