It discredits feminists with the left given that the left is supposed to stand up for the vulnerable, the dispossessed and oppressed.
This is begging the question. People on the right would argue that the right wing is better for the vulnerable, dispossessed and oppressed, and that it's the left wing which oppresses people. You are pre-supposing that "left = good, right = bad" and automatically taking issue with any argument that contradicts this.
but then I didn't think I'd have to spell out the power and economic disparities between men and women which make prostitution possible.
I'm all for reducing poverty among women. Reducing poverty among women means that women who don't want to be sex workers can stop selling sex. I don't think anyone would argue with you that reducing poverty among women is a bad thing.
Pope Francis calls it the torture of women, quite a difference stance from the previous religious view that prostituted women were "sinners" with nothing whatsoever to say about the torturers (johns). If Pope Francis can see it why can't you?
Pope Francis's comment about tortured women was regarding women who find themselves in a situation in which they don't have a choice. I don't think Pope Francis would hesitate to call a woman a sinner for having sex with a man she wasn't married to, if she had the choice not to do so.