Sanemi is a math teacher in an academy, and giyuu is a P.E. teacher in the same academy (Also, the other hashiras are teachers. I just don't know what teacher they are exactly)

Giyuu and Sanemi have a secret affair that no one knows about, not even other students, because everyone thinks they hate each other, and they honestly don't think sanemi would be good in a relationship which is not true, he actually acts like a gentleman when in a relationship even with his anger issues and he always got flustered by any small action of a word of affection that giyuu did or said

After a long and tiring hour with his students mostly with inoske and zenitsu but not tanjiro because he knows Giyuu wouldn't be really happy if he yelled at him.

Sanemi was sitting in his office taking a small smoke break in there until Giyuu walked into his office Giyuu made sure no one saw him going there and then closed the door and sat on Sanemis desk right in front of him

Sanemi looked over at him and used his stern voice in case someone was at the door

"What are you doing here? Shouldn't you be? I don't know teaching some of those little shits?"

He said, his voice still stern well. At least he kept it until giyuu confirmed that no one was behind the door listening to them.