A flat out ban on PDAs in general just makes it easier to control, since there's hugging and
hugging. But what do I care, I went to an all girls school and I didn't have to witness any monkeybusiness.
PDAs in general... well I agree with Avarion about the tongues. Tongues aren't pretty.
And as for the people in my age group now, PDA in excess is often related to how insecure one person is in the relationship. They continually need to stamp ownership on the other person. If you're the recipient the clinginess can get really irritating. I'm sure some people like it, but the ones I've known have gotten really tired of it after awhile. In a couple of cases it's been a dealbreaker. Let's just say that no one wants to be groped in front of their mum/dad/grandma.
Imo, there's a time and place for things, and if you're sitting down to dinner in a nice restaurant that isn't it. Your friends probably prefer some conversation over watching you play tonsil hockey.