The "Devil's Highway" refers to a girl's butthole. Most guys only get anal sex when their girl had about 50 drinks. I mean, she is so fucked up, it's like the devil has taken control of her. We all know the devil does some dirty shit. See example.
So when you're banging your girl in the ass, your taking a "Ride down Devil's Highway." For those guys fortunate enough to have a girl who actually likes anal, marry those women!