I did have some shots taken of myself and my girlfriend of the time having sex but that was for a fine art photographer (that's not just what he told me, I went to his exhibtion!). It was a really interesting experience and despite my reservations at first it really just ended up as my girl and I doing something very intimate together around a bunch of people that we barely noticed being there.
When it comes to porn I don't think I could do it though. between the performance anxiety, the stranger(s?) that you are having sex with and the continous directions from the guy making the film I would imagine that there are sewage plants with more of a sexually charged atmosphere.
It's kind of interesting, I like porn, I use porn, but the differences between pornography and sex in real life are vast.