Though I get your meaning, consider that in the real world, most of the choice acting roles BY FAR are written for men. Actresses (or female Actors if you prefer) in Hollywood are always complaining about not being able to find really good roles. And basically, if the role isn’t that great, or the movie itself isn’t that great, you’re not going to get nominated. I mean, let’s say they made Sisterhood of the Travelling pants with Merryl Streep, Glenn Close, Judi Densch and Cate Blanchette, and all 4 of them turned in the best acting job of their careers, we wouldn’t be seeing them in the best Actress category.
So, because of this, it would really be an unfair advantage for men in the “best Actor” category if men and women were running against one another, because most of choice roles in the best movies were written for and played by men. I’d venture a guess that you’d see men nominated 3 times as often as women and probably would win 4 or 5 times as often. Then THAT would be considered sexist.
It’s simply a function of who runs the Academy and the society they grew up in. For example, I had in my first year of College, a Freshman Comp teacher who was probably in his early 60s at the time, and I wrote a paper about something with current relevant culturual impact…this guy was very much into being proper about your grammar, so I was very aware of how I put things down on paper for him to read. So, my paper was about the success of a particular movie, and I referred to the female lead as an “actor”, because this was the late 80s/early 90s and at the time, there was a push to call all actors, male and female “actors” instead of using “actor” and “actress”. That was about the only thing he corrected on my paper, he crossed out actor and wrote “actress”. I explained to him the modern trend in our language and he just wasn’t aware of that. It wasn’t a sexist thing, just his whole life, that’s how we’ve referred to people who act, using gender specific words. I’m assuming that this is just how most of the people who run the Academy are used to it being this way, with best categories for each linguistic term…it’s not an inherently sexist thing, and overall it actually results in women getting more awards, so I don’t think it’s probably ever even occurred to the powers that be to consider changing the way things are done. If it ain’t broke as they say, don’t fix it.