And who decides those circumstances? Okay, I agree that certain circumstances might lend itself to certain behaviors, but even that is more subjective. More people would agree that a behavior is unacceptable if it causes harm to others.
Boils down to what some people find offensive or immoral, not just harmful.
You're hanging out with some buddys at a bar, you see two dudes in the corner kiss, who gives a shit right?
You're sitting down eating with your kids and the two dudes in the booth across from you start kissing....YOU may not find it offensive, but some people might. The circumstances are different and some people would expect a little decency in public displays of affection.
To answer your question, society as a whole decides those circumstances, as well as what is immoral or offensive. And to be quite honest, the majority of society is silent on these types of subjects so as not to be labeled by the minority that screams the loudest.