I'm generally ok w/ PDA, as long as it stays within the PG/PG-13 realm. Kissing and neck-kissing and hugging, and some tight squeezing and such, but no tit or groin squeezing. Families should be able to take their kids out into public w/o having to explain that shit every step of the way. If you're gonna have sex in public, do it so you don't get caught (even if it feels like you MIGHT)!! In addition, for a wide variety of reasons, most people just DON'T wanna see you have sex. Most guys just look gross, and many women do, too. It's wild to imagine seeing some hot pornstar-type gal getting fucked when I'm walking through the park, but the reality is usually different. If my wife's with me, well, I wouldn't wanna look very long, and if my kid's with me, well, that's a problem, obviously. And odds are the woman won't be so hot, PLUS I won't be the guy fucking her, so really, it's just kind of annoying in addition to the other issues... It is nice to see people expressing some affection, though - just not sexual foreplay (or sex).
That said, PDAs are much better than PDV - public displays of violence.