Sensei probably knows Otoha forced herself on Niki, he points this out before trading nudes in Otoha's 'Taint the sapling' Chapter 4 Event:
Sensei definitely doesn't like the idea of any of the girls with another guy. He mostly just considers them being with another girl as hot, though. Keep in mind, he's kind of a manchild that just sees girls as fleshlights deep down.
Strange enough, Yasu never really mentions the roof. Then again she seems to have a White Room and her god's eggs that help her survive resets.
It may require tragedy and love. Ayane wasn't exactly loved by Sensei until she lost the baby, and found out he lost his memories, which is when he finally accepted they were dating (shortly before Ayane survived a reset).
Makoto is similar in that it wasn't until after she jumped, and lost her father, that Sensei finally made it clear that he loved her, shortly before Halloween, and again during Halloween in front of Miku, then she survived.
Ami just went through a tragic period, may have been murdered, more or less broke, and now Sensei seems to be willingly saying he loves her in front of others.
Previous Ami's may have just never been broken or loved enough. Not to say this one has.
There also seems to be some godly permission required. Pareidolia seems to approve of Ayane, and something else approved of Makoto. Maybe also a sacrifice if the lives of Ayane's chicken Todd, and Makoto's father, were necessary.