Women's/Gender Studies people - what's the difference between primary and secondary sources?

I really only know the difference between primary and secondary sources within history, when it’s often easy to tell which one a source is (and then other times, I’m totally confused, and think that the primary/secondary/tertiary split is inaccurate). But I don’t really get how to apply that to things outside of history.

So, for example, I’m doing pornography within the Feminist Sex Wars. So, Dworkin’s Men Possessing Women from 1979 is, I assume, a primary source. But then Catharine MacKinnon’s Only Words in 1993, which draws heavily from and sides with Dworkin? Or Nadine’s Strossen’s opposing work Defending Pornography in 1995? What about Pat Califia’s essay Among Us, Against Us – The New Puritans: Does Equation of Pornography with Violence Add Up to Political Repression? from 1980, only a year after Dworkin’s Men Possessing Women? In history, usually they’d all be primary, because they’re happening at roughly the same time, they’re contemporaries, etc. But then I’ve also heard (from someone in my Feminist class) that what divides them is if someone is writing about someone else; so if Patrick Califia is writing about Dworkin, then it’s a secondary source.

I ask because I need to come up with a thesis that includes what kind of sources/evidence I’ll be using, so I need to be able to say if I’m using secondary sources or primary sources or both.