I'm curious what moral distinctions (if any) exist between, for example, the Venus de Milo or the Birth of Venus and "soft" pornography like Playboy?

Well I'm definitely no expert here, but often in our moral evaluations we take into account intention/motive (as perhaps one factor among others). And if one might arguably hold that "art" aims for some kind of "higher" purposes beyond merely sexual stimulation/titillation, then at least those works of art have some kind of additional value beyond their attractiveness (while, perhaps, the "soft porn" aims only for the stimulation....)

Catherine MackKinnon and Andrea Dworkin were among those feminists who lead the charge against pornography, so to speak, calling it what amounts to a central pillar in the edifice of patriarchal oppression. However, humans have only had free and easy access to visual pornography for the past few decades, maybe the past century, at most; and even literary pornography, like Fanny Hill, only dates back to the 18th Century. Before this, the inability to mass-produce pornographic materials seems to imply that it can't have had much the same impact as it could potentially have today - and many such depictions were on urns, floor mosaics or other objects with further purposes, so they weren't there strictly for sexual arousal. Yet it doesn't seem unreasonable to say that Western civilization has seen women forced into the status of second-class citizens for many more centuries than that, even millenia. So how can pornography be the central factor, or even a central factor, in the oppression of women? Or...

A fascinating issue. I'm not familiar with the specifics of MacKinnon and Dworkin, but I'm not sure that the considerations you mention would necessarily undermine their thesis, as you stated it. For whatever the history, it may well be that right now, these days, pornography plays that role (assuming that's what they argue for, of course), even if other factors played more significant roles prior to relatively recent technologies. Or it might be that ultimately their status as sexual objects has reflected and/or caused their secondary status all throughout history, and the contemporary technologies serving pornography merely continue (and exacerbate etc) that same trait. No doubt anyone concerned about the status of women in western society will begin their investigation historically and find plenty of explanations for the oppression of women; at the same time, any attempt to emphasize one, or give it central prominence, risks undervaluing the others .... best, Andrew