This is not the weirdest or grossest movie, which I've ever seen. It's just "Japanese". The guy doesn't tell to his boyfriend, what's his problem, but the boyfriend of course knows there's something is not fine. And when he realized that his boyfriend has a hair loss, he just started to lick his hair while he's having a bath...
I think the message is about no matter what's your illness is, I love you the way you are. It's sure an absurd way to express your love to your beloved one, but the essence is the expression of love.

And if I think of the movie like this, I don't think it's bad. The amateur camera usage is much rather problematic. Like a home made video... It's not acceptable for a movie, no matter how short it is or how beginner the staff was. That's basic.

Otherwise, who's that person, who never did anything weird with their beloved person in their private sphere? It's just got a publicity.