When a performer asks me what their character's motivation is, I tell them that their paycheck is their motivation.
I'm all for plot and character development when I sit down to watch a 2-hour movie created by a large crew of trained movie-industry professionals and performers which cost millions to make (especially when I know in advance that no hardcore sex is going to appear on screen), but realistically I know that is not what most fans are looking for when they are in search of porn (we call a cumshot the "money shot" for a reason).
This does not mean that I shoot gonzo, or advocate throwing out all production values, since I try to consistently strive to improve on the quality of my amateur(ish) productions. Most of my work is plot driven (albeit with quite simple plots). I do this as much for my own enjoyment as for the audience. I feel that if I am happy with a piece of work, then others may be happy with it as well.
ADG