Isn't it obvious? I mean, if you think about it for a few seconds it should be. People don't like being uncomfortable. People like to feel safe. People don't like unpredictability.
If you're frenching, it seems likely you could do anything, strip nude on the spot, etc. There are videos online of people doin' it in the middle of supermarkets...
It's all about respect. If you have respect for others, you won't make them uncomfortable. If you are disrespectful and make a public display of yourself, expect people to think you're rude. You're being rude.
Do you yell in front of people you don't know? Or do you quiet your voice? Why? Out of respect. You know you will alarm those people that don't know you if you start to yell - for whatever reason -. If you do yell out, do you not expect people to look at you with alarm?
If people are holding hands, and then they start feeling each other, and then they start french kissing, isn't it natural that they're going to keep progressing? Why would you even french kiss if you WEREN'T going to progress to the next step? You're just getting yourself excited for nothing...
The phrase get a room is adequate. You obviously want one!
Obviously, this is a cultural thing. If people are lounging all over Mexico frenching all over the place, far be it from me to judge. But in my country, it's a weird thing to do.