My, eth0, you're quite the perceptive sophisto...!
Just teasin'. Yes, the goils are paid, and they've signed model releases, the whole shabang. On some web sites, the hosts actually (claim to) state how much each model charged for her services. The going rate for a scene is $300 to $700 at an "amateur" site.
This is one of the oldest plot lines in porn, way back to the early days (the 70's and 80's). Random boy meets random girl. Within three minutes they're clothing-free and he's balls-deep. Yeah, right!
Rock on...