Here's what bothers me about the type of PDA under discussion:
1. They guy usually has that "here comes another fatherless baby" look about him.
2. I often get the feeling that it is some sort of "prove your love" bullsh!t; "See how much I don't care about the other people, honey!" Strikes me as insincere (unless it's in the U-Bahn or a Park at 2:30 on Saturday night and they're just drunk)
3. The active pair often have the gaul to give you that "what are you looking at" look!
Of course, most of this is just my gut-feeling, which is hard to back up in any structured way.
I think taking pictures is a good come-back. Also, if the couple just moves off the main path against a wall or into a corner, it helps loads. Shows a bit of respect for the rest of the world.