Well they were probably just poking fun at'em; if anything they probably figured they could get him to scream out that he doesn't want his sexual relationship with Nishimura to be illicit, for all to hear. A bit of embarrassment for the normies, but still a little shipping for'em by their friends in the process.
Of course it won't be illicit unless they get into some really freaky shit when they're older, so he's got nothing to worry about.