We humans have been hardwired by evolution to feel and behave in ways that, on average, were adaptive in the past. As we spent nearly 98% of our evolutionary history in small groups and life was most definitely a zero-sum game (your gain being my loss, and vice-versa) we are adapted to compete for what used to be scarce resources. Although we now live in a very different world, our behaviors are hardwired and we can't escape them.

So what does this have to do with the disapprobation of other women toward those in the lifestyle? Simple: those other women see a threat. And threats must be countered by whatever means are available. As women are typically less practiced at combat, violence is largely verbal/psychological rather than physical. By attempting to cast the lifestyle female as "immoral" etc. ordinary women are attempting to reduce the risk of "their man" being "poached" by a "predatory" interloper.

This is the same reason that comfortable middle-class women are the most ardent opponents of decriminalizing sex work: they pretend to care about the welfare of sex workers but all their words and actions are directed towards minimizing the potential threat, not toward actually ameliorating the lives of such women.

It's always instructive, when observing human behavior, to (a) consider our evolutionary history, and then ask (b) cui bono?