Corrections for the above spoilers because it's off.
I didn't want to say anything because reading speculations is interesting, but honestly, Saitou is getting too much hate. Saitou tells Isechi to give up because she knows that Sensei is so nice that she would let herself get dragged along (even after rejecting Saitou, Sensei was still looking out for her). Saitou is under the impression that Isechi will eventually fall out of the illusion of love (just like she did) and in the end, Sensei will be the one to get hurt.
When she was pregnant and met Sensei again, the conversation was Saitou's way of telling Sensei that she's moved on, she's fine now so Sensei shouldn't have to worry about her again. Isechi then swoops in to claim that neither of their love is an illusion, that she'd never give up on Sensei, and that she wants to believe Sensei did love Saitou and now does reciprocate Isechi's love.
Saitou then apologizes to Sensei, thanks her, and makes it known that she is indeed happy with her husband and child.
Left out some details, but that should clear things up.