Of course it's assault. Just like Luke raped Laura on General Hospital. Just because someone doesn't press charges doesn't mean it didn't happen.
Will Smith is one messes up dude. Poor guy has to let his wife sleep with younger men just so that she will keep his secret. Even if Jada is a good beard, he still needs to get out of the marriage and come out. It's 2022, people will still love you no matter your rainbow connection, Will.
He actually started laughing at the joke, but when he saw that look on Jada's face, he knew he was going to get a beat down from her if he didn't macho up and do some asskicking.
If (for example) Lupita Nyong'o's boyfriend did the same thing Will did, he would have been escorted out of the buidling and charged with assault. But Will is too rich and powerful. Different kind of justice for someone as rich and powerful as Will Smith.
And of course Rock is not going to press charges. He will lose the respect of many in the Black community if he does. Plus, he probably realizes that it was a shitty thing to joke about (and the joke wasn't even that funny.)
How about the bit with Regina Hall sexually harassing some of the men in the crowd? Not very Me Too of them. How would it have looked if it was a man doing the same bit?
