if a man wants to fuck a woman he could use this expression. it implies he has a large enough cock and a hard enough boner he could "rub her belly from the inside" when he fucked her.
hey babe, wanna a belly rub
When your dog lays down on his back and sticks his belly out for you to rub! Gotta love it, it's so cute! :)
I luv my wittle Murphy!