Oh god I thought I was the only one who felt this way
I think it's a wide area between rape tropes/feminine ukes/masculine emotionally void semes and this manga, whose characters were essentially Daria without the funny. acerbic wit
It was a cute little story but for as long as it was, not much happened. You can have light fluffy manga without making the characters lifeless.
I've got to agree with you. The characters were bland, which made me feel like I could not care any less about them. A reader needs emotional investment... well, this reader does. It was cute, sure, but so is "See Dick and Jane" and this manga had the same depth. Also, does this really qualify as yaoi? I saw the tip of a dick (censored to boot) and a bare ass. Everything else was implied. I guess the dick makes it yaoi, but just barely. Without that frame, we were firmly in shounen ai territory.
I've read oneshots with more character development. Seiichiro especially was just... nothing.