Femstarion would be popular because she would introduce herself as promiscuous. There would be no misrepresentation or misleading. People who like experienced women would sign up from the start.
The problem with Shart is that she explicitly says she's not the third wheel type in act 2, and then later if you ask she reveals she's actually been daydreaming of having sex with Halsin for an extended period of time, and disapproves if you don't have sex with him. So it feels like she was misleading you earlier in the game, and most players who signed up for a usual waifu are surprised by her behavior that contradicts her prior words.
I think ultimately Shart would still be the most popular waifu even if Astarion were female, because more men want a traditional waifu over an experienced waifu, though it would be relatively close. Most normies are not far enough in the game to learn that Shart is actually a poly queen, so the same situation that exists right now would still exist.
Female Gale and female Wyll are both interesting too. Female Wyll would be insanely popular. The traditional virgin princess who wants to marry you and dances with you would be a smash hit.