Was Marx a feminist? Did he back women's lib? Did he like women's lib as much as John Stuart Mill? These are questions we've been discussing a lot in my graduate seminars. Here is my answer: it doesn't really matter if Marx would have championed feminism (a term that didn't exist until the 20th century, after his death). We have no way of knowing if he would have liked this concept or not. We know he didn't invent it in his writing, which deals remarkably little with women's issues, but just because Marx didn't talk about women's issues as much as John Stuart Mill, the all-time icon of liberalism, doesn't mean Marxian discourse can't be good and feminist.
Sure, Marx didn't think of women more than the average academic of his day, but just because he wasn't privileged enough to be around for the era when feminism took off and actually became a term doesn't mean he wouldn't have been the biggest pro-feminist man the world has ever seen if someone had been around to explain the concept to him. He also might have hated it, saying women are by definition different and inferior to men, and calling them equal is absurd. We don't know. But then again, Mary Wollstonecraft wasn't around when the the term "Feminist" emerged, either. Yes, her writing expresses a concern for allowing upper-middle-class white women to work and vote, but for all we know, if she existed today, she might be one of those annoying chicks who say stuff like, "I love my glamourous job in advertising and all, but feminism just goes too far. I mean, like, I totally like having sex with men and I wear a bra!" We don't know if she would have cared about contemporary feminist issues like race, class, sex workers' rights, and so on. We don't know if Mill would have cared about these things, either. We don't know and we can never know, but, as I am about to argue, that is not the point.
It doesn't matter if historical figures would have identified with us (ie. self-identified feminists). All that matters is whether or not we identify with them. Sure, Marx wasn't really around to weigh in on feminism, but we feminists can weigh in on him. We can appropriate Marxian concepts like dialetic materialism and surplus value to understand our own world. Standpoint theory is actually largely influenced by Marx's theories that the proletariate, as they see both how the middle-class lives and know the drudgery and oppression of their own lives, have a better, purer knowledge than the middle-classes who are separated from the means of production and have no idea how the proletariate lives.
When we write a piece, we cannot be entirely sure who we are representing until the world weighs in. We must see who relates to it first. For example, Gloria Anzaldua's writing about a hybrid identity inspired by her Latina roots and their interaction with her American present, actually really register with me. My background is incredibly different. My moms WASP and my dad's family is of Armenian descent, but I still feel like "wherever I go, I carry my home on my back," just as she says. I feel homeless, diasporic and blended, like she does. Anzaldua was probably not thinking about ANYONE at all like me when she wrote her feminist theories, just as Marx wasn't really thinking about women, but I see in both of them theories that register with me as a person and as a feminist.
So, when writing feminist histories, and teaching feminist history courses, the question of which thinkers and figures to include or exclude should not be driven by whether or not we think they would have self-identified as feminists if they'd only been alive at the right time. There is no point in assigning identities to people that they themselves might have abhorred. Instead, all you need to do is decide, as a feminist, whom you relate to. If it's part of your feminist thoughts, it can be valuable to feminism, regardless of whether the person who wrote it was an official, card-carrying feminist.