The "mewling quim" bit is actually in the midst of a pretty solid character development scene for Black Widow. Whedon is relatively great when it comes to the development of female characters, and I would not even argue that his use of the phrase indicates any sexism on his part. It merely highlights the profound sexism of our patriarchal* society. (* - Oh no, I used another word that will trigger (OH NO AGAIN!) the @Darji crew.)
Black Widow is a good character told very well by action movie standards.
Nothing you mentioned involves those galdarn feminists calling for censorship or retconning. People have some opinions you do not like and it goes absolutely no further than that.
There is nothing thrown at any of the males by Loki that matches the tone of "mewling quim". I fully admit that this is because there is nothing Loki could possibly say to the male heroes that would match the tone of him calling her a "mewling quim". Men are not objectified and marginalized by their sexual organs the way women are. (Imagine, "Fuck off, you're just a male." It doesn't even work. It makes no sense, but "Fuck off, you're just a woman." totally does.)
His manipulation of Hulk is specific to the characteristics that make him Hulk. His manipulation of Black Widow is to just dismiss her based on gender. This isn't me feeling sorry for Black Widow because Loki said something mean, it is me realizing that a 13-year-old girl is watching this film and seeing her very body used as a generic insult while the boys get customized attacks.
Yes, there is a relationship between beauty and sex in certain contexts. That was not OP's point. OP's terrible and completely wrong point was, and I quote, "There is nothing wrong with “male gaze” – on the contrary, it is only through the perspective of a man (or a lesbian, I suppose) that female beauty can be fully appreciated and celebrated."
You are not arguing the same thing as OP.
Black Widow is essentially called a cunt in Avengers. No other character is derisively objectified, but that is partially because there is no way to do that to a male as you do a female. It is harsh in a way that the rest of the film is not.
I am the proud father of two young daughters, so it is completely terrifying. It is horrible to put out the idea that a person's beauty can "only" "be fully appreciated and celebrated" through sexual desire. That's how "you're just a sex object for men" perpetuates.
No doubt that sexiness and beauty can have a certain relationship in a certain context. I don't think anyone is denying that.