There was also some theory about short-haired girls, if I recall correctly.
Was it, "short-haired girls are cute"? That's my personal theory.
I'm pretty sure that's a universal fact. It's one of Sappho's Three Laws of Gayness:
1- For a yuri manga's frame of reference, a lesbian protagonist shall either stay in the closet or drift around looking for cute girls at a constant pace, unless acted upon by another lesbian force.
2- For a yuri manga's frame of reference, the overall gayness of the plot G is equal to the uselessness U of all the lesbians involved multipled by the thirst T that they accumulate over the story: G= u t
3: When one lesbian attempts to flirt with another, the second lesbian displays a level of gayness that is inversely proportional to the length of the first lesbian's hair, until their hands end up equal and opposite each other.