BTW, I pulled up the comeliness rules. They're heterosexist, not sexist--heterosexuality is assumed, but the sexes are treated equally by high-comeliness fascination effects, and all references are to "he or she".
You are right that sexist might not be the best description.
I think it is the specific descriptions of how people specifically react to specific levels of attractiveness that is off putting.
Unearthed Arcana page 6:
"+ 14 to + 17: Interest in viewing the individual is evidenced by those in contact, as he or she is good-looking. The reaction adjustment is increased by a percentage equal to the comeliness score of the character.
Individuals of the opposite sex will seek out such characters, and they will be affected as if under a fascinate spell unless wisdom of such individuals exceeds 50% of the character’s comeliness total."
***
"The fascinate-like power of high comeliness is similar to the 2nd-level illusionist spell of the same name.
Those subject to this power will be captivated by the user, and treat him or her as a trusted friend, mentor, and companion. A saving throw versus spell will negate the effect but if the comeliness is not magical in nature, then dispel magic, antimagic spells, and similar spells will not affect the fascination effect.
Fascinated creatures will follow the orders of characters with high comeliness, provided a roll of 3d6 does not exceed the comeliness of the character. Requests that are not in the best interest of the creature get a + 1 to the die, while those that are hazardous can gain up to + 6 or higher on the die roll. If the roll is higher than the user’s comeliness, the fascinate-effect is broken."
This is not something I necessarily want to be rolling for and roleplaying this specified way constantly on either side of the screen. If it had just been a number to represent abstracted attractive looks, or just affected the reaction adjustment mechanics it would not have been as distasteful.
Mostly Basic and AD&D left roleplay type of stuff up to the players for their characters and to the DM for NPCs with a few minor mechanics for charisma affecting hiring henchmen and an often ignored reaction mechanic. When magic affects such things like a charm it is a vancian resource a PC intentionally expends with an attached saving throw, not a constant effect that will impact lots of people every time you walk through a city. This was an unwelcome escalation of "play this way" focused entirely on attractiveness interactions.