Think about how you have sex in your private life and how sex is portrayed on screen. You (and I) would think that the goal of filming sex would be a voyeuristic one, candidly capturing a real and natural encounter. On a porn set, this can be far from the case. There can be lots of people around, the director is calling the shots, and the whole thing becomes mechanical.
Personally, I've never had an erection problem when it comes to anal sex (can you say salute, Sergeant Prostate?), but it can get pretty damn uncomfortable if not done right.
Watching a behind the scenes segment with Billy Brandt, the two guys are nonchalantly going at it. The director says, "Let's see some good facials," and their response is magically transformed.
I'm curious about your comment "Especially when they cum." Are you talking about when the top cums, or do you see the bottom cum with a limp dick? I'm scratching my head on that one.

