The really great thing about agreeing to only fuck a girls ass (because she is not protected for pussy sex and enforces ass-fuck contraception), is that you can always manipulate things to finish fucking her asshole when she’s lying on her stomach.
Why is this “really great”? Well… you don’t even have to try to poke your cock in her pussy and thus risk her rage. If you’ve done your ass-fucking job properly, when she holds her ass cheeks apart for you to come in her gaped asshole; more often than not - she’s so horny that her pussy gapes as well.
It’s really great to shoot as much of your ‘hot gooey seed’ into her gaping pussy as into the gaping ass that she thinks is the only available target.
Your cum is very adept at seeping into vaginal depths. Your little swimmers will do their best to sneak past her cervix and invade her womb. Go team Y-chromosome.