I've not met anyone who doesn't laugh at couples who display public affection in the same way they laugh at toothless people, people with one shoe, teenage dads, etc.
Ie, down their nose.
It's just a white trash behavioural trait that warrants mockery and derision.
It doesn't even need to be pornographic, frenching in a park, even excessive back-rubbing while waiting in a line, it's all embarrassingly lower class behaviour.
No one I would ever consider associating with would ever consider engaging in it, or not sneering at it when they witnessed it.
I've actually dumped a girl who tried it on me, and I knew she was trash before I started dating her but I was in denial because she had a nice body, I could only maintain that denial up to the point where she revealed she's the type to attempt public displays of affection.
That was it. That was the low point, akin to the alcoholic waking up in a kindergarten covered in vomit with his pants down, I had to confront the fact I was dating trash and take the appropriate steps to correct the situation.