I was spanked as a child (although my mom developed eczyma at some point and her doctor told her to use a slipper -- that was the end of spanking being useful as a form of discipline - ha ha). While it didn't traumatize me in any way nor seem like sexual abuse, I can say that I don't see why hitting your own child is a good idea. There are more humane ways to discipline other than hitting.

But whether it is sexual abuse or not... I think it depends upon the intention, not the act itself.

(edited to add trigger icon)