Is public display of affection rude or not?

I was a the metro, coming back from school. I sat down listening to my music. Then a young couple (people around 20) go into the metro wagon. I said like O.K their a couple. Then they looked around quickly and saw that the wagon was crowded, then they whispered something at each other (listening to music didn't listen to convo). A moment later they are kissing lightly. Then they take a break, look around and then kiss lightly again. This went on for 10 min until they got out.

I'm single. I feel that one Light Kiss is an acceptable thing to show Love. But more than that it's screaming insecurity.

So Is PDA Rude or not?
PDA = Rude
PDA = Acceptable
Select gender and age to cast your vote:
Is public display of affection rude or not?
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