There are things about anal in porn that I dislike, but I like the act itself, though I prefer straight vaginal. A2M disgusts me. It's obviously unhealthy for the chick, and from the guy's point of view -- how on earth could you attempt to kiss the girl afterwards?
Double anal (and vaginal) is a massive, massive turnoff too. I've seen about 5 seconds of each and instantly switched off. Not only does it seem like an obviously bisexual sex act (strike 1), the poor girl must be in absolute agony (strike 2). I can't stand any type of group sex with multiple males (strike 3).
There's no hint of chemistry or attraction in those scenes for me -- they feel like National Georgraphic, the people come across as nothing more than rutting animals. Unfortunately for me, multiple male scenes seem to be increasingly prevalent in mainstream porn.
I don't necessarily need to always feel that the girl has to be the 'hungry, crazy & wicked lover' as jamrak said. There can be something sexy about a girl being in a little discomfort, coming across as a submissive 'victim'.
The first time I had anal with my gf, she was coming across as shy, innocent... she was in a little pain, but wanted me to carry on, and she also wanted to feel dominated. I enjoyed that as much as the times she has been the hungry lover, perhaps even more.
A little pain can be hot -- it comes with the territory -- spanking, pinching, mild biting, whatever. There's a line that shouldn't be crossed however, and that's when discomfort and slight pain becomes obvious agony. The little I've seen of double anal and double vaginal places it in that category. The very thought of it seems like an extreme female version of a kick in the nuts.