Butt slapping happens in locker rooms.
If you're slapping a woman's backside, it's much easier to sell that it is about sex. Or maybe I should say, it's much harder to sell that it ISN'T about sex.
Watching the video, it looks like Beckham wanted/expected a reaction, but it wasn't about sex. It was about him being dumb and horseplay.
Doesn't make what he did right. It was dumb.
But I definitely think it's dumber to slap a female's backside than a male's. Too much metoo stuff (with good reason, I suppose) going on to get away with that.