I always cringed when I grew up seeing woman's asses getting smacked. The thing a woman's curve and butt is one of my favorite things (legs, butt, eyes, hair and face).
This is such a gross violation.
This is all about power not sex.
Now if a person is in a consented relationship and they both agree it is ok ... smack away.
In my house, when I am bent over doing the dishes or doing something, it is very common to feel her hand on my butt. She tells me what a great ass and then laughs ... of course your mouth replicates it too :)