1) PDA's are unacceptable because people don't want to see anyone snuggling, or kissing. It makes many people uncomfortable and may upset those who suffer from the lack of a significant other.
2) Couples can do what they want. If you don't like it, you don't have to look at it. It isn't their fault you're uncomfortable with it.
I'd like to know what people have to say regarding each of these arguments, or if they have other arguments to add to the mix.