I've heard that, when asked about it, Gene Roddenberry (who wrote the story), admitted that the whole "no women captains" was simply sexist. I can't find anything to support it on line, but I also didn't look for that long.
My sister is a Trek fan and a staunch feminist. This episode was always one she never wanted to watch after the first time. Recently, I asked her to watch it again, but just watch it for Shatner. I said, "whatever you may think of the episode or his choices, I find him mesmerizing." Later on she admitted she was able to enjoy it on that level. It's hammy and stereotypical, but he's body language and vocal control is incredible. He's a lot of fun to watch and he did it while battling a bad bout of the flu.