Neutered dogs and bitches are just as likely to hump as entire dogs......it can be play, excitement or (when it's between two dogs/bitches) sometimes it is a display of dominance
Molo used to hump his towel when I was drying him; he has stopped more recently, though
It is perfectly normal dog behaviour, and so should not be any more "embarrassing" than barking, or scratching
I am certain that Tia isn't at all embarrassed by it