I've seen damn little SPN het, fr'instance, that actually uses a character you could call a Mary Sue.
Most of the Supernatural het I've seen could just as easily have taken place with a headless female body. The characterization of the female partner, if she be not canonical, is often the very last priority.
Of course today I saw a couple of anti-slash gen writers (absolutely anti-slash, not just gen-preferring) boggling at the concept that there could be some latent misogyny in slash circles. Not just boggling, mocking the very idea.
OMG I saw that too, and I rolled my eyes so hard. I couldn't even formulate a response that would have opened their eyes, so I just filed the participants onto the Stupid List, and closed the window.
I think they're more from the 80s/90s era, and such slashterpieces as A Fish Called Krycek.
Actually, now that you point that out, I can theorize a little. If they molded their slash goggles in that era, then it's entirely possible that heavy exposure to the het OTP KGB made them so violently allergic that they've been overreacting to het ever since. I certainly know that at that time, in that fandom, slash could occasionally be the antidote for the flavor of extreme domestic normativity that infected segments of het. (Unlike most fandoms since, XF's huge audience included huge numbers of people who never had, and never would again, be in fandom -- and those people, in a mass, could be really overwhelming and stiflingly heteronormative, gender-normative, etc.)
But, like, been there, read that (wrote that), ate the t-shirt, still like girl parts.
Your therapy process, let me show you it.