I think it has nothing to do with sexual behavior, and the fact that people attribute "physical disciplinary action against children who exhibit bad behavior" to "sex" is just mind-boggling and weird. It's probably just a coping mechanism against childhood trauma or something, considering otherwise it doesn't really make sense.
Don't get it, just not my thing! But you can still dig your nails into my back if ya want to!!
I like to include it sometimes assuming she gets something out of it (no fun spanking a girl who is completely unenthusiastic about it).
On a little side note it does tend to make me a bit more aggressive and dominant for a while. Or caring and dominant, depending on her reactions really. Hard not to be the one in charge after giving a spanking.