Both the Basic Rules and Cook/Marsh's Expert rules were filled with non-cheesecake illustrations of female characters of every class. Now, it's not perfect. The illustration of Morgan is pretty ridiculous, and both the female magic-user on the cover and in the illustration mentioned above sport high-slit dresses that show off some skin. But the book as a whole was inclusive enough that I could give it to my 12 year old sister, and she was immediately onboard and eager to play.
This continued with the 1983 Mentzer revision. Though he abandoned the explicit "he or she" style in favor of a slightly more stilted one that avoided third-person pronouns, the interior artwork by Jeff Easley and Larry Elmore was an improvement even on B/X with its inclusion of tasteful and inspiring artwork of female characters. Again, not perfect. On the plus side, it introduces the cleric Alanna as a mentor to the (illustrated male) tutorial character. On the minus side, it fridges her not long after her introduction. But this was all pretty progressive for the early 80s.
Now, given the totality of TSR's history, from the 1970s through the late 80s and early 90s, why were these products comparatively non-sexist? Because this was peak "D&D as mass market product" era. TSR was happy to be progressive if they thought there was money in it.
1985 saw a withdrawal from this as sales plummeted in the post-fad era. This was when Clyde Caldwell, with his Neo-Frazetta style, was hired, and put in charge of the Gazeteer covers. 2nd Edition, while being much less cheesecake in the core books, elected to use "he" throughout the PHB and DMG. It seems to me that with falling sales, the company endeavored to market more towards what they perceived as their core market: young, male hobbyists. And ironically, this was all under the auspice of Lorraine Williams, with Gygax ousted in late 1985.
My sister and I got into D&D in 1987, first with B/X, and then BECM, so our exposure to TSR cheesecake was somewhat delayed. From our experience with BECM, we were fans of Elmore, and regarded Caldwell with some exasperated amusement. In all his art for Dragonlance, Elmore drew Goldmoon as wearing long buckskin pants. When Caldwell drew her for DL 1 - Dragons of Despair, he naturally removed her pants. We understood then that Caldwell's desire to draw female skin knew no logic nor shame.