This is a bug.
Apparently, after a player-initiated same-sex relationship has happened, the game is supposed to make most sims straight and some sims gay/bi/ace/aro.
However, until that happens the percentages are borked and it actually does the opposite, making the majority aro/ace. By default all sims were meant to be pan.
This post explains:
https://answers.ea.com/t5/Bug-Reports/OPEN-Townies-with-no-Sexual-Orientation-at-all/m-p/11735371#M228240
Might also want to navigate to page 1 and Me Too it.
Workaround:
https://answers.ea.com/t5/Bug-Reports/OPEN-Townies-with-no-Sexual-Orientation-at-all/m-p/11736365#M228267
Or you can use my method and roll their prefs via a dice app whenever you make over your townies. You lose the surprise a little that way, unless you have terrible memory like me.