It's actually up to your interpretation as to whether or not it was a happy ending. You're definitely in for the most angst and tears you'll ever have to endure by reading this, but it'd be SUCH a shame to not read this. It is worth it. While yes the two MC's DO end up together, the seme is a total piece of shit and it makes you wonder if you really WANT them to be together. The only reason it's considered a "happy" ending is because our baby, Junichi, finally gets his happiness with Kazutrash. The side story has a bittersweet ending at its purest. I won't spoil that for you since the ending is a big conflict within the manga, whether or not Kenta can feel romantic feelings for Gakkun. It feels bitter, but the sweet there is also undeniable.
Guys does this have a happy ending, but that I mean a real good happy ending? cause I wanna read but the comment section is full of tears and I don't think I can handle tears and angst.. TT^TT