First, let me be clear that I'm not talking about a guy putting his hand on his girlfriend's ass to signal possession. I'm talking about the more "I wuv you" types of PDA, along with things like bringing her roses at work (a disgusting site I witnessed the other day in Starbucks). Obviously not all girls look down on this servile behavior, but for those who do -- the majority -- what are they anxious about?
A female's health and age can be easily seen by just looking at her from afar. But other qualities, such as the many personality traits, are more difficult to figure out that way. Even after interacting face-to-face for awhile, you only get a crude estimate of how worthy she is on these other traits. But one much more honest signal of her hidden qualities is who she associates with -- in general, birds of a feather flock together. The literature documenting assortative mating is pretty large by now; for example, people tend to pair up with partners of similar intelligence, height, age, and so on.
As in many human relationships, there is a conflict of interest between the boyfriend and girlfriend. If the boyfriend is the type to lavish PDA on her, he is most likely a loser (relative to other local males). He is desperate to announce that he managed to punch just a bit above his weight. Why? For one thing, others will elevate their estimate of his quality if they see him getting away with PDA with a girl who they wouldn't expect him to get. It may also allow him to punch above his weight several more times, as female onlookers who are about as good-looking as his girlfriend will pause and think, "Hmmm, he doesn't seem so great, but if she is with him, he must have something I can't see." This is why men always resort to PDA, boxes of chocolate, roses, etc., when they are most desperate -- this is when they are most clearly reminded of how far out of their league their girlfriend is. And of course there is the hope that pleading will work.
The girl for her part wants the exact opposite. Clearly the partners will not be exactly equal -- one will be a little above the other, since the cost of searching and landing a partner of exactly equal value would not be worth the benefit, compared to finding someone who's close enough. Still, if she's the one who's above him, he may want to gloat about it to the whole world, but she will want to keep it a closely guarded secret. If her friends and acquaintances found out that she was "dating down," they might lower their esteem of her, and she would suffer the resulting social consequences. This is even more true if there is no enforced monogamy -- then, she may also be concerned that potential future mates will see her with a desperate man and think, "Meh, she looks OK, but if she's with that clown, she's not worth investing in as a wife."
The end result is what looks like a lonely father trying to smother his baby girl with hugs, while she violently pushes him back and orders him to stop embarrassing meeeee.
So are there pockets where PDA abounds? Clearly if the girl is deeply in love, that's as good as enforced monogamy, as the potential future mates are completely out of sight, out of mind. But these are rare, since usually a girl will not fall madly in love with someone far beneath her. If there is greater social enforcement of monogamy -- say in more religiously conservative areas -- then guys will be able to get away more with PDA. And in places with greater civic engagement and altruism, there is less ruthless calculation among one's friends and acquaintances -- they aren't going to jot down every little signal of your hidden qualities, because they're more trusting. So girls would be more accepting of PDA in a small Minnesota town than in Manhattan.
Which of these two forces -- enforced monogamy vs. peer esteem -- is stronger? All we need is a case where they run against each other. In middle and high school -- and even somewhat in college -- girls are subjected to heartless scrutiny of their inner qualities by all of their peers, not just their friends. For them, gossip and drama are constant background noise. On the other side, monogamy is strictly enforced -- not just by her friends, who would cast her out as a slut if she dated around, but also by herself, as girls are much more susceptible to falling hopelessly in love at that age.
Being under her peers' social microscope predicts that she'd want to shove her overly affectionate boyfriend away without even thinking about it, while the power of monogamy to cloud out her thoughts about potential mates predicts that she wouldn't care. For anyone who's been to the park or the mall in their lives, the answer is pretty clear -- monogamy is stronger than peer scrutiny. There's no mystery why a girl would allow PDA from a handsome quarterback, but a lot of the guys who these adolescent girls are with are pretty awkward and goofy. The fact that they accept PDA from them is pretty startling, so it must be that their more monogamous behavior, compared to 20-something women, outweighs their anxiety about what their peers will think when they see some doofus wrapping his arms around her.
It may be hard to remember, but monogamous behavior at that age is not driven only by social or institutional pressures and falling in love more easily. There is also the mindset that all reckless young people have -- it's going to last forever. I don't think you appreciate how strong that delusion is -- for every year since they started middle school, teenagers have seen their friends come and go as though it were a game of musical chairs, and yet they still use the term BFFs to describe people who they'll drift apart from the very next year, or at the latest by their sophomore year of college.
Natural selection must have designed the human mind this way to make sure that in the period leading up to starting a family, the partners -- who have potentially very different interests -- would trust each other a bit more. Few would stick it out through the first years of raising a family if one of them thought the other would quickly defect due to peer pressure. So a monogamous impulse had to evolve to be even stronger. Starting around her mid-20s, a female no longer has the same force of social scrutiny weighing down on her (people become more independent then), and she's more OK with dating around or practicing serial monogamy since she'd be past the typical age at first birth (the early 20s). It's only then that they start to really resist PDA across the board.
(Much later on, when monogamy is de facto because no one else would want her, and as she becomes even more socially independent, then PDA is allowable again -- surely you've seen those revolting scenes among middle-aged married people at the table next to yours in the restaurant.)
I don't like PDA because, again, it reeks of desperation. I don't mind picking girls up and spinning them around -- that's like showing her that you can open a jar that she can't, or drive better and faster than she can. However, if you're the gushy type, we can tell where you'd find more accepting girlfriends: find a young girl in a religiously conservative area where people are more trusting than calculating.