For those who may not remember, I doubt it's simply Io's trauma that has made her Anti-sex. As she's pointed out, even without her trauma, things wouldn't work out with Sensei:
Presumably, Io is anti-female due to trauma, but also asexual anyway, so sex itself isn't romanticized by her like it is by most. She just sees it as repulsive and unnecessary:
The reasoning behind the throuple is more or less that she wants nothing to do with sex:
Realistically, this isn't changing anytime soon:
It'd probably be easier to make Osako straight or Sensei homosexual.
There
is always the possibility that Io gets desperate enough to just drug herself unconscious, then let Sensei do whatever, as long as he cleans up his meat juice, or something, but yeah, if Io just starts wanting sex, she's probably been replaced by an alien or a meat puppet.