After the initial filming, performers of both sexes -- and not always the ones who did the acting -- add what's known as an "m-and-g track," for "moan and groan," a separate audio track of rehearsed expressions of rapture that are overdubbed -- that explains all the times the movements of her lips just don't match up to the "Oh, baby."
I think I have found my calling in life. This would be so fun. I could say, "No, I'm not a porn star, but I play one vocally."
Now I know you all want to read it, but won't admit it, so I will post a link.
Cowgirls and Gigglers: Girl Porn Slang
Hammer Jones (My porn name, if you follow that pet/road convention)