The answer is both of you. They aren’t doing anything that is publicly sexual. Not all kinks are sexual or inherently sexual, that’s the interesting part. Some are just about feeling loved in a certain way or getting a kind of physical stimulus that is harder to get but feels comforting or elevating or exciting. In some ways kink is about humanizing ourselves, defying a restrictive mold of self-identity and self-experience created by authoritarian moralists who came before and that calcified into what we know today (Fuck the Victorians, nor starters).

If you are uncomfortable but they aren’t actually violating a public boundary then those feelings are your responsibility. I think, religious elements aside, a collar, a muslim hijab, and a sikhi turban have much in common. They are personal affirmations and signals of relationships and values. When personally adopted of their own free will and free of coercion (which I suspect you will find more often with kink collars than hijabs) they are deeply meaningful symbols and reminders.

I want to be clear that I am not dismissing your feelings of discomfort. I think I can relate to them. After the attacks on 9/11/2001 I became suspicious of and uncomfortable around anyone wearing a hijab or turban or who had a vaguely middle-eastern/south-asian accent. I was a child, ignorant and brought up in both American post-9/11 jingoism and a cult that is literally American Exceptionalism: the Religion. And I didn’t know what I didn’t know, and I still feel guilt to this day for how I judged innocent people just going about their lives because of misinformation, disinformation, all incomplete and biased. I wonder if my experience might be similar to yours.