Backstory for the seme? Wasnt it explained already? He liked uke since he was kid, tried being the best "older brother" for uke, thought it wrong to have feelings like that for a kid who treats him as an older brother(as any decent person should) so he tried to suppress his feelings by going out with the girl. Uhm isnt that enough? And isnt being honest and trying to be a better person for the other, character development? Uke stopped sleeping with random guys then seme decided to be honest with himself and told uke how he feels. I do agree on your thoughts about masaki tho hes kinda meh. Also what he did was soo out of character it was unsatisfying.
I know that there was backstop for the seme but it was just so sudden and appeared out of nowhere. Its like the author just wrapped it up in a page or two about his love for the uke. Thats why it felt super rushed to me.
I thought that the story was solid, I mean it has been done before. But there was no character development or back story for the seme so it was pretty funny to me when he suddenly said he loved the uke lol. Also, I get Masaki is hot but he didn't really... do much for the uke either. I guess the characters in this manga just didn't have any development and felt superficial af. But it is a good read for something to pass the time!