I don't think explaining to you is going to help, but I'm going to try. The word "bitch" is gendered and the gender is female. It's also a word associating with the degradation of and the mistreatment of women. It's a word used repeatedly to undercut and dismiss the arguments of women.
To say someone is "Bitching" or a "Bitch" is to suggest they are saying something of little value, but because the word is gendered and negatively affixed to women, it's misogynistic.
I'm not the one who has to catch-up on my slap, it's you.