I'm sorry, the 3/5 rule was designed to give MORE power to slave states by counting the slave population as part of the census for purposes of alloting Congressional representation, even though, obviously, slaves were disenfranchised. It resulted in the votes of white men in slaves states as counting for more, because they received greater representation, than the votes of enfranchised voters in free states.
Yes, slavery was contentious at the Founding, but its wrongness was clearly not as universally acknowledged as it is now. I think people who are comparing moral intuitions now and then are saying that there has been, for the most part, a move in Western nations since the Enlightenment toward MORE equality, MORE representation, MORE inclusiveness in regard to "who count" as a fully participating member of the larger society, and that feminism is a part of that movement. More people now recognize women as fully participating members of the social and political realm than did 200, or 60, or 30 years ago. Even the people on this thread who espouse a contrary view for the most part include a line about "of course" wanting equal pay for equal work - but this was by no means obvious even 30 years ago.