Yes, personally I don't see someone's sexual orientation as a threat. But in Japan, (please correct me if I'm wrong), especially in a setting like the education field, being gay is a big NO-NO. Parents will be concerned that their kids interact with a homosexual in school; faculty members may look at Kai with an unpleasant gaze. Worst case scenario, Kai might get complaints from parents and eventually get fired. That is why someone's sexual orientation might be used to blackmail. But, this manga is pretty light and cute, I doubt anything would happen, and I'm intrigued to know what decision Kai will make in this situation.
In conclusion, Japanese society is not so tolerant of open sexual orientations. I know that time has changed, many people have opened up their minds and are less judgemental, but there are lots of people who hold strong opinions against things they don't understand. This is true regardless of which country we are talking about.
Changes can occur on the surface level, but in order for people to truly change their opinions, time and knowledge is the key.