That's the point! We're all saying the relationship is cute but in truth, the two characters are both quite twisted. For instance, consider Yajima's previous relationship: he knows his partner has a wife and a family but he doesn't even take take that into consideration but simply wishes that his partner would reciprocate his feelings, even if it's only minutely. That kind of amoral attitude towards cheating, posessiveness, preference for middle-aged men, and desire for kinky sex is hardly something that's perceived as normal, especially in Japanese culture.
But because of Watabe, who also turns out to carry unorthodox views, their relationship works out. For instance, Watabe relishes Yajima's posessiveness, finding it adorable. And because they're both fond of each other's oddities, the relationship ironically becomes cute and healthy.
Manga is good but the guys are way to weird for each other but I guess that's what happened when u fall in love for the first time ...good stories both I like it but to each there own