It's not like the guy will have any emotional attachment to sensei like MC does, and she is a married woman and clearly not a virgin
Maybe this is exactly what she needs to open up to a possibility of cheating on her husband with the MC.
Shes not a whore and she seems to genuinely like the mc, even though shes married so she shouldn't be doing it in the first place, but shes not immediately opening her legs for the mc even though she clearly harbors some sort of feelings for him, so I find it hard to believe she'll have sex with some guy shes just met and doesn't even know and is a gigolo. Especially when its taken a long time and a lot of persuasion from the mc to do anything at all. But this is manga logic so...
It's bait, since she is just doing it to make the MC jealous for eyeing out the girl that totally wasn't in his apartment, she was eavesdropping and showed her frustration known when the new assistant mentioned that. we'll have a chapter from her perspective while she is eavesdropping and getting jealous again, saying yes to the guy to get MC jealous, though I don't understand why she would want to get him jealous really, it's not like she wants anything from him.
Now saying all this I'm still expecting the ntr like I have from the beginning and was and am prepared drop it if so. More power to you if you like it buts if it comes to it and the ntr does happen it's not really my cup of tea.