Love, Lust and Lies: There is a time and place for that, but not in public
Most people think women are the instigators of PDA, when in reality men are just as much to blame. For guys, though, it has more to do with being territorial. They want everyone to know that a woman is "theirs." They might as well pee all over her leg like dogs do — that will for sure keep other guys away.
PDA is not always a bad thing — some forms are tolerable — so I've written a list of types of what is acceptable and what isn't.
Hand Holding: This is the most common and most acceptable form of PDA. There is nothing wrong with going for a walk in the park or walking to class holding hands; although, if you're out with all your friends, do you really think you need to hold your boyfriend's hand? It can make your friends uncomfortable when they see your hands all over each other.
Kissing: There are acceptable and inappropriate kisses in public. When you are out in public, the only time that should be acceptable to kiss your significant other is when you are saying hi or bye, with a few exceptions for thank you kisses, all of which are brief. Let me clarify, just because brief kisses are acceptable forms of PDA does not mean you should be doing this all night long. If your kisses linger and it turns out into a groping session, please refrain from doing this in public ever again. Seriously, no one wants to see it, and if you can't be out in public with a person without always having the urge to jump his or her bones, maybe you should stay home.
Pet Names: Nicknames are never okay in public; no one needs to know you call your girlfriend "Snuggle Bunny" or boyfriend "Teddy Bear." Those should be private. I know it's hard to avoid the occasional "babe" here and there, but try to keep these to a minimum.
Personal Servant: The worst kind of PDA is when you find girls or guys doing everything for the other in public. For example, when you see men holding their girlfriend's purses, not because their hands are full or they need them to hold it for a minute, but because they are whipped. Seriously, grow some balls.
Personal Space: Sometimes in relationships you need to give each other space when you are out in public. The perfect example is going out to eat at a restaurant. Nothing is more awkward than seeing a couple by themselves but sitting on the same side as each other. That never made sense to me; you can't even talk to the person without twisting your neck — how is that comfortable? You don't need to be side-by-side 24/7 when you are in a relationship; space can be a good thing.
There are some forms of PDA that are tolerable in public — you don't want to act like you don't know the person and are afraid to stand near them, but you need to understand that your PDA makes people uncomfortable and, yes, even jealous. A basic rule of thumb I like to follow is if you don't want to see other people doing it, then you shouldn't be doing it.