"knocking boots"

"That's always struck me as an odd term for it" my fella A___ said the other night.
Seems he wasn't clear on why it is a word for sex. And so now we have a dilemma. Urban Dictionary offers it's usual variety of user given definitions. The key word being "variety" there. After I demonstrated to A____ what my conception of the origin of the term was (seems I'm a freak as I quite naturally assumed it was the sounds your boots or shoes made when you had sex with them still on), A____ explained (with lively gestures as well) what actions he figured the term might refer to. They included "when someone goes to take off their pants and they step on their shoes to get out of them fast..." which sort of loses something without the visual to go with it, and the action where one person's legs are wrapped around the other person's hips during a penetrative moment. Presumably, the penetrated person's feet would meet on the other side and knock.

But it strikes me that these may be silly. Any other thoughts or definitive definitions, or should I keep browsing through Urban Dictionary?