A COC forces you to be compliant to it every where on social media, on regular media, in person, on private forums... everywhere - this is thought police.
A COC forces you to "hide" your personal beliefs, ideas, who you vote for who you donate money to etc. on the off chance they might be considered offensive to some one.
We already had a CEO who had to resign due to a political donation, COC's will have the same effect you post something offensive on facebook? you're out, you go to a political rally that some one finds offensive? well though luck.
If COC's only covered official project channels no one would care about them, and the people that want them would not push with such vigor because they could not be used for thought policing.
On a side note I find it ironic that people who had to hide who they are for most of history and maybe for most of their lives want to force the same thing on other people just because of their personal beliefs.
And on another side note the wworse of the humanity tends to fester in dark rooms when things are out in the open you know here everyone stands and you can ridicule them for that if you want.