Love

What exactly do they say in that situation? I suspect you are referring to an English language idiom though not necessarily a popular one at that. I can't think of a suitable idiom that would apply with that scenario that involves horse riding.

As an English person i'd tell them to;
-Man up
-Tap that ass
-Butter that crumpet
-Smash the gash
etc.. (if it rhymes with a synonym for vagina it will work as an English phrase)
(( In fact, if it relates to any sort of motion it will suffice i.e. I'll motor her, steamroll her, pendulum swing her, ride her, etc...))

or if you want one for the actual act of going to her & also related to horses you have;
- Blaze a trail ( act boldly and make a move, based from the wild west)
-Sweep her off her feet (Based on what Knights riding horses did)

nothing else springs to mind.