You're post was about 10% sarcasm, 85% insults, and 5% debate.
I'd say any adult interested in watching films on a regular basis would be annoyed at the mere presence of a child while watching an adult film. Kids are noisy, kids get bored and run around, kids are messy, and if the film is scary the kid can REALLY bother the other people watching the film!
People have made the point that adults can be just as bad. Well, adults misbehave in pubs and clubs, don't they? But pubs and clubs (most of them anyway) don't allow kids, even if they do have parents with them.
For the kids that aren't as good watching horror as other kids, watching the film at home is alot better. In the cinema, you got that big screen, you got the loud surround sound. Heck, sometimes the noises make me jump! I've seen adults nearly fall out of their seats because of loud unexpected scares!
A theatre screen showing an adult movie with kids watching it is more likely to have people screaming and being annoying than a theatre screen with no kids.