For clarification: When I say they function outside of the relationship I don't mean things like MC acts this way because of outside circumstances which is an understandable path for a character to take but when the writing applies that trait towards how MC treats the ML and makes decision within his relationship it's not "functioning" outside the relationship at all.
An example would be Jinx, We have our MC, the money and his grandma. These are traits that make up his decision making throughout the series along with his personal feelings for ML. You cannot say that these outside circumstances means MC functions outside the relationship when these specific traits are what kept him in it.
Which is why Happy Together is so precise in its relationship. Seungmin has nothing to do with the band, It's simply circumstances that the drug he's hunting down traces back to Jaeyoung's which a thing we call, Ladies and Gentlemen — an effective premise. Outside of their feelings for each other and affection, Jaeyoung has outside relationship that he can also focus on, the same with Seungmin. None of those outside relationships are consequential to their romance. That's what makes it different.

> Happy Together
> Rating [5/5] ☆☆☆☆☆
There is something to appreciate about the way this genre has manufactured and capitalized on characters as labels. If you need a recommendation you can simplify it into two words, #AdjectiveBottom and #AdjectiveTop. I have for the longest time found series that I enjoyed and loved in this way and is by no means bad nor an indication of quality.. sorta until it borders into caricatures.
To simplify what I'm getting at if it isn't obvious yet. Happy Together breaks that ceiling, Will you label Jaeyoung as #QuietBottom or will you label Seungmin as #CoolTop? Many have added nuance to characters they've written throughout my time with this genre but none come close to feeling like real actual tangible people. Jaeyoung isn't defined by his role in the relationship and not does Seungmin. They're defined by the actions taken upon them and actions they're taking for the betterment of the other.
We see them joke around, function sexually like actual couples and most of all.. they are intimate in ways NO OTHER COUPLES I've seen have been. There's been dialogues yes, a showcase of profound change after being an asshole but no other series makes it explicitly clear that these people care about one another beyond their attraction.
Seungmin and Jaeyoung feels like they function OUTSIDE of their relationship as a couple, It's a huge plot glue don't get me wrong but they're not centered or defined by it like most other series are, Their relationship is a portion of the things that make who they are and the actions and decision they make. Jaeyoung isn't going into this because he's feeling things for Seungmin, He's doing it because he genuinely loves performing and Seungmin isn't working against Jaeyoung's autonomy with his wishes and plans.
This is only the surface, I can go on and on about how stellar the pacing has been for a weekly release, There's a rhythm to it that feels almost cinematic and not just panel-y and that takes incredible precision. The writing is phenomenal and each side characters feel just as alive as the leads. It's a small thing but when most of what you read here feels like the same thing in a different coat, Happy Together breaks boundaries and shows the quality we could get from serious artistic finesse.
You root for these characters not because the plot tells you to but because there's risks involved and there's a possibility of a setback that can come from an interaction with the overarching antagonist and you will not get that on any other series because most of the time.. what you read in BL? The ML is as much the villain until the story needs him to become the lover instead.