I have no idea what Takatora thinks about sex, but I'm sure he doesn't think Michi having it. She's too... responsible and studious, if a bit hesitant to bother him about things (like getting sick from cramps). He might look things up and see that it is birth control and even for Michi, it would be a good idea to have some backup.

(Safe to say, it wouldn't end well for Kouta if Takatora found out that they were sexually active, even if Michi initiated it.)