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....)
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 ....