who decides what is or is not acceptable?

There is no one person who has decided this. As a culture, norms have evolved. If you are interested in the development of norms, then you have to read up on cultural anthropology. Different cultures have their own norms. Few norms cut across all borders. A physical relationship between mother and son is one of those. Oh, and a lot of times religious institutions have a role in what is good or bad.

a physical relationship between mother and son?

ok, inc.est or s*x if you prefer?

Well it’s sort of common sense and respect. Like, yeah aaaaaages ago somebody probably decided that it’s bad to spit at people or whatever, but you know what… it IS bad to spit at people. Rights and wrongs truly are RIGHT AND WRONG. I guess we have evolved through the ages just ‘knowing’ what is right and wrong. Like, cellphones have only been around for 40 years or so, but already the world has developed customs and acceptable/unacceptable behavior around them. Like you don’t start texting when somebody is talking to you, don’t use it at the table etc. It’s just common sense and politeness.

Many of the words that are considered “bad” today, were actually words used in Olde English in regular conversation. Language has loosened up considerably since I was a teenager….when I was in highschool, the word f*ck was NEVER used by anybody….out loud, anyway. Now, it hardly turns a hair on anybody. Words move with the culture of the times…

Society.