looking for yaoi manga where the relationship of heterosexual couple (side characters) are developed as well as the main couple

not fake already read it

thanks

"you realise that the idea of hope is purely a construct of your mind. Hope is not a tangible object. You cannot have hope. You can only pretend to have it." - you find yourself in a room

Not to get your hopes up with this reply, but I think FAKE is the only title that does this in any substantial way, at least that I know of. There's a little bit in Gravitation with Hiro and that other girl, and major characters who are married, but they don't get nearly as much attention as Yuki x Shuichi.