I think Sana just doesn't want Sensei to be off limits. Even back in 'The Next Best Thing' Chapter 2 Main Event, she claimed:
Sensei ending up with anyone, kind of makes it hard to make him her fuck buddy. At least, that's probably along the lines of what she thought back then.
Now, she seems to think he can be shared:
I think the kind of relationship Sana is most interested in, is probably the one she seemed to have with Sensei in the recent reset puzzle:
Seemingly the kind of relationship Sensei and Maya once had, hence Older Sana becoming Maya in the 'Remember to Smile' Chapter 3 Main Event:
And just like how Sensei seemed to call over Older Sana for sex a lot, he seemed to have texted Maya for sex a lot:
Kind of a fuck buddy relationship, albeit, I think Maya wouldn't allow him to be shared at all. I'm just waiting for New Maya to find out Sensei is fucking someone else. That'll probably answer a lot of questions about her. It doesn't seem like Ayane has mentioned his other girlfriends to her.