It really depends on what it is and your partner. Some people see a request for a threesome or opening the relationship as the end of the relationship.

I personally don’t know if I’d continue a relationship with someone who asked if we could do scat stuff or piss play, especially if it was an open relationship.

Some requests could be too close to previous trauma, like consensual non-consent. Some could lead to a loss of respect, like adult babies.

That said, there is the question of whether you’d want to be with someone who would break up with you over your kinks, so I wouldn’t say you shouldn’t ever bring it up. I just don’t think it’s realistic to expect that the worst that could happen is getting a “no”.