I've felt this for a long time. Girls in porn seem to have very much less trouble taking it than guys. I think it is because of their make up down there.
I've asked female friends and they say there's just a thin wall between the rectum and vagina, and in a porno I saw a guy stick his finger in a girl's ass and press it up so you could see it in the vag.
That front wall in a guy's ass is pretty solid.
That being said, I've never seen a girl cum from a dick up her ass like a guy can.