It's been said but I think the reason, in art, is that it's easier to get a good shot in for what's going on. Once the bodies get all mushed together all the sexual bits get blocked and obscured.
I think men are infatuated with seeing a long thing enter into another thing, especially if it's soft and wet. The more length that can be slowly pushed into the soft wet thing the happier they become.
I bet it would be heartbreaking for men to learn that penetration isn't all that great for a woman and it becomes a mental game of getting into it enough to get off, but nothing beats just rubbing or vibrating a clit. Once you do the act of penetration and if you move, that subdues the clit sensations and doesn't get you to that mind blowing orgasm as much.
I've had vaginal orgasms, but they are done with concentration more than feeling unless the mood is absolutely amazing. Clit rubbing though works almost every time and doesn't take as much effort to get there.
All those long dicks I think are ironically long to please the men looking at dicks more than the women taking them, artistic or otherwise.
. Or the AVN focuses 99% on sex with "internal shots" or so XD
