I have been reading about John Stuart Mill and his views on equality in The Subjection of Women. It seems that an important part of his argument is that equal rights for women will benefit mankind/society as a whole. For example, it seems that he condemns sexual inequalities of opportunity by appeal to the social utility lost - so men are harmed by any inequality. Does this mean that he is not a feminist?
In general, does it matter for feminism that a major motive for equality is the benefits to society as a whole, rather than specifically being concerned about the oppressed sex?
I think that in light of the analysis you have given Mill and what you have taken away from The Subjugation of Women
, it would only endorse the fact that he is a feminist. Keep in mind that feminism is above all a reaction to inequality where the goal is that women must attain equal legal and political rights, as well as address the relationship between the sexes, the inequality, the subordination, the oppression, etc. Feminism is in many respects is the medium through which we should identify those sources of inequality. John Stuart Mill is great in the respect that he is taking utilitarianism and applying a feministic lens through which we see the feminine disparity in our own social normative framework. I think that in Mill's case, utility holds the most appeal if all gears are working in tandem rather than having an inherent friction in the greater machine by means of sexism.
With all that said, it would matter a lot if feminism were geared towards equality in terms of the whole social framework. That seems to be (at least as far as I see it) a fundamental motive of feminism. If feminism is a reaction to disparity in rights, etc. and seeks to equate the sexes, then as an end goal, it would matter whether or not the sex in question were actually able to contribute equally to the social framework as a whole. In many ways though, this is concern for the oppressed sex, but only so far as it relates to equality in society. If you look through the writings of Betty Friedan or Simone de Beauvoir, you see a great deal of reactionary sentiment that turned into revolutionary sentiment. 3rd wave feminism is tricky like that because it gave rise to radical feminism and so on where the disparity and equality goal faded into a more drastic counter push, becoming more severe than the subtle male normative framework supplied for many years past. 1st and second wave feminism are more in line with the fundamental definition of feminism, as well as those principles extolled by Mills, Wollstonecraft, etc.