I disagree. Best Girl chooses borderline femboys both because having a "Soul that burns brighter" correlates to being effeminate and because her sisters are attracted to feminine males. Given Best Girl's aspects are "Life, Growth, and Fertility" it makes since that her influence has effected their soul's development. She also chooses them because she knows her sisters will go easier on them, leaving themselves more open. If she summoned someone ugly bu strong they would still not be able to simply overpower a domina (Even Horace was considered attractive by them).
I don't get the impression that she put that much thought into it. It's more like she picks boys who are to her liking because the idea of a boy that could tickle her fancy overcoming all the girls who were mean to her seems romantic but she's not entirely aware that this is her logic and so tries to dress it up as a "can't be helped, this is the only way!" sort of necessity that, as Mary pointed out, isn't the case at all. And Mika was also wondering why Llavanya never chose a woman to be a champion. You're supposed to be thinking about why she chose to do things this way because the Dominas themselves are wondering and the answer seems clear: she's not particularly bright or a practical sort, she just really wants things to happen a certain way and became detached from the whole process because she never had any bones in the game.
And she probably doesn't realize that as a Domina she's sadistic too. Watching beautiful boys march into hardships and suffer in her name kinda does it for her. It's kind of a 'secret slut' thing, too, you see it when she tries to say that she believed in all of her champions. In these types of stories it's really supposed to be a chosen
one and that's either coyly or outright framed as a sanctified union between goddess and hero, but she's giving it up to all these guys, most of whom she cared so little about that she didn't even bother introducing herself.