Yes, I know the women were overtly sexualized in the Original Series. However, if I'm not mistaken, the outfits were a concession to the network, and in the original pilot (The Cage), the women were dressed in pants that were the same as those worn by the men. Gene Roddenberry had no choice but to "sex up" the women if he wanted his series aired. Even still, by the standards of the time, the Original Series was quite groundbreaking in its treatment of women (Uhura on the bridge, for instance), and in the movies the women wore the same uniforms as men.
He aspired to a world of gender equality, and later incarnations of the franchise should have taken that further and made good on the ability to do what he could not, instead of selectively confining themselves to the boundaries set by 60's network executives whenever convenient.
I have never read anywhere that the network forced Rodenberry to "sex' things up. If you have a cite, I'de be more than willing to check it out.
Gene was the creator and executive producer of TNG (one of the later incarnations of the franchise), and that show had catsuits on all crew members and included the womanizing Riker, with his bevy of "the willing". Troi was the one in the little mini skirts when she wasn't in her catsuit.
Gene certainly had a vision of equality for women, no doubt, but I don't think it included turtlenecks in space.
I don't have anything that explicitly states Gene Roddenberry was forced to change the women's uniforms as a condition for the airing of his series, but I believe it's what occurred because, in part, it fits in with what we know did occur and it adequately explains the change from pants to skirts.
For example, we can also point to the fact that Majel Barett originally played the Second in Command, which was found to be too radical for network executives. We
know that Gene wanted Majel as Number One because he intended to "have a woman play a character in a clear authority position" but changed it when the network wasn't comfortable with a female second in command. Nichelle Nichols even
claims that she was originally auditioned for the role of Spock, who was meant to be a female character in early drafts. It seems completely reasonable to conclude that a roughly concurrent change in women's attire was done for the same reason.
I may not have conclusive evidence, of the kind that would pass as beyond a reasonable doubt in a court of law, but I think it's a completely reasonable extrapolation of what we do have.
It is true that Deanna Troi was originally dressed in rather feminine attire, which would change to a regular uniform in TNG's final seasons after Roddenberry's death, but I believe that was because her purpose on the Enterprise-D was an embodiment of the sort of "touchy-feely" sensibility that was common in the 80s. Her open uniform was suitable as part of that role, and better suited it than the stiffness of a regular uniform. After all, Dr. Crusher, Tasha Yar, and many other female characters on TNG wore the same uniforms as men right from the get-go, as they did in the TOS movies.