Staring is nearly always impolite whether it is at smoochers or not. We should not give others undue attention. But there is brief public kissing and there is making out, whichfeels like a prelude to something quite private... and we are all wired to find watching others exciting! See the popularity of !$%*! Research activation of "mirror neurons". We naturally feel either jealous or awkward. Poor Dilbert....!
In my experience on both sides of the situation, it's only impolite when the person/people witnessing it doesn't have a significant other to kiss.
Though I will admit I still think people making out in public for extended periods of time is classless and kinda icky. I doubt that's Dilbert's problem here...
It is still unpolite!
Don't you feel awkward it is. If someone is kissing you cant watch that way. If multiple cuples pollute the public you can soon not look anywhere but straight down to the ground.
One especially extra-awkward situation if is you are visiting a couple and they start making out while talking to you inbetween. That should be punishable by death!