ridin' the horn
Performing a sex act similar to ridin' the rod; the dude is on his back, so his erect cock sticks straight up like a saddle horn. His lady aligns her pussy with the ol' woody and sits back in the saddle formed by his crotch and his legs...then she slides up and down his cock, slowly at first, then faster until he's buckin' and fuckin' her ass off...and when they come together, the ride is worth it!
"Hey, man, I wouldn't mind takin' one of those cowgirls for a ride...you know it's cool to be the saddle when she's ridin' the horn."