imho a highly disingenuous way [not you Number 6, the guy you quote] of saying the obvious-but-kapu:  women are considered less than fully human in a male-supremacist ideology, thus to "treat someone like a woman" is the ultimate put-down;  also, in that ideology, the "correct" way to keep women and children in line is with physical bullying, hitting or threats of hitting ("discipline");  and notions of control and dominance even in the abstract are likely to be verbalised through metaphors of rape, prostitution, and battery.

To open handedly slap someone. Denote disrespect for the person being bitch slapped as they are not worthy of a man sized punch. Suggests the slap was met with little resistance and much whining. Urban Dictionary

Explananda -- I find the commenters' bewilderment about how this expression went mainstream a bit naive;  it went mainstream at about the same time that everything else about prostitution went mainstream, over the same period when yuppie college kids started going to halloween parties dressed as "Pimp and Ho," and yuppie moms started buying their 7 year old daughters "Porn Star" brand underwear.

Wikipedia -- different threads emerge here, one being that a "bitch-slap" is the kind of slap a girl would deliver, i.e. not a real punch.  the dominant thread however seems to be:  a slap delivered to a female by a dominant male, to shut her up or subdue her.  at any rate a highly gendered term.

could it be the author is baiting his [male?] readership by saying, "hey, the Republicans are treating you like women, you are their bitch, where is your pride?" -- without any reflection apparently on the degree to which his comment accepts and reinforces the same attitude to women?  kind of like Red-baiting, the ground assumption being an inherent ineradicable Taint (of femaleness, of Socialism) whose ascription would galvanise the readership into an  reflex of repudiation.  the original form of "if you're not with us you're against us" being, "if you're not masculine and tough then you're a girl, eee-yew, cooties."

"rough" slang in this case seems to me a euphemism for "pimp-speak" or "overtly misogynist cant" -- imho.  to my ear this is as regressive as e.g. promising to "work my new trainee like a n*gger", with many of the same implications of dominance, coercion, contempt.  would we laugh off such language as "rough slang" (with its connotations of rugged, if slightly oafish, masculine virtue) or recognise it as simply racist?  and does the writer resort to this "rough slang" in an attempt to retrieve, and flaunt, his own Liberal Masculinity in competition with the Rethugs who have recently attempted to corner the meme-market in Manliness (even if a wee bit o' padding is required here and there)?

I was discussing with an old friend -- old enough to remember, as I do, a time when prison rape was something that shocked and disturbed average Americans rather than being treated by tv comedians and ordinary people as a kind of national joke -- the curious shift in attitude to this abuse.  he said

I think there's also a class thing going on here; I remember listening to Kris Welch's phone-in show on KPFA in the early 80's, right after a couple of really loathesome child molesters had been caught, and hearing a caller, after carefully noting that she opposed the death penalty, go on to speculate hopefully that they might be killed by other prisoners. The moral cowardice of this, and the peculiarly class-based attitude--that we will let the lower orders do the dirty work while we keep our hands clean--still makes me mad. (Kris, a person I've always thought was basically dull-witted, supported the caller's sentiments.)

there may be some connection here to the economic draft, and the sending of lower-class boys (and a few girls) off to do the dirty work of Empire (the 'wet work' as the spooks call it with rather awful accuracy)...

The difference between theory and practise in practise ...