Sometimes you can't help how you feel. Everything is fresh and new so you just can't keep your hands off each so therefore you express it to your surroundings.
PDA is the one thing some people might consider it being one level above actual public sex. When you are in a new relationship, everything feels so good and spicy. You are having make out sessions everywhere, cuddling, fondle each other, and of course hold hands. Once you got used to each other it might narrow down to holding hands or a peck on the lips. It's something that people can have fun with and play around with especially when your relationship is at its highest, but how much is too much PDA?
Most of the time the line gets crossed depending on who's around you. Some people go by the moral rules of PDA vs. what people think is the respect level of others. As far as morals, some people think that if you are going to perform PDA, then it clearly shouldn't go too far to the point where you need a room because for 1 it's inappropriate and 2, it looks disturbing to others. As far as respect level, some people feel that if you are going to french kiss then make it quick and don't eat each other's faces in front of everyone. For others if you are going to grope each other, don't stick your hand down the other's pants in front of everyone. Of course most couples could care less about what other people think about their love, but there is a line drawn at some point.
Some people think that it's not so much the actions that crosses the line, but the place. When people see an affectionate couple especially a new couple it is kind of expected that they are going to perform some sort of PDA. The actions can be as explicit as possible just as long as other people don't notice, but depended on the place some people might get offended. If it's like a church, in the middle of a street, or doctor's office or something those are considered an inappropriate places. Places like a bar, park, or a subway are more acceptable places.
Whatever a couple does with each other is their business. If they feel like their PDA is equivalent to public sex then that's what it is, just respect the people around you. Couples are going to do what they want no matter who is around them. Sometimes they do it either to make someone jealous or to impress other people that they like each other. It's nice to see couples be affectionate with one another even though it can fizzle out later in the future but as long as the love is still there, PDA is whatever they want it to be.