I think I would find it personally distasteful, and I think it also
walks a fine line for me, but I can't really come up with a logical
reason for it to not be OK, so I would have to go on the side of not
having a problem with it.
While having a slave on a collar may be an expression of sexuality in
a way, it's not people having sex in public. If I kiss my girlfriend
and my boyfriend in public and we snuggle, it's a declaration in some
ways of what it is that we do in bed. A person on a collar is more of
a declaration of what the people are doing in bed than an explicit
showcase of sexuality.