I don't know about you, but there's something about filming a sex scene that would always seem awkward to me if I was one of the actors.
This is a prime example. Demi Moore is show, rather explicitly, giving Michael Douglas head. I don't know how he could've film that scene without achieving an involuntary erection on camera.
Hehe. emailtom's answer was really funny.
Well, I think that in this kind of scene where 20 or 30 people are surrounding you it must be difficult to get a hard-on. Also actors usually perform scenes many times so the excitement gets old.
Besides, by the time they both hear;
"Scene 137, BJ Money Shot...Take 32"
...even if Michael Douglas was 18...it's somewhat...acedemic...
Many anti-fur protests, few anti-leather. Seniors & models must be easier to bully than bikers.