I don't think Garnet is a representation of a married couple.
I've seen where some will call Garnet relationship goals. That she's so well-adjusted, that she's a beautiful pair of women who have learned to work at love so well it stopped being work.
Garnet is not a married couple.
Garnet is the child of a married couple, and a perfect parallel to Steven.
Steven is the result of his mother giving up her physical form in order to create Steven. Steven is, broken down, a fusion of his parents. His mother's love of life and adoration of the universe, and his father's humor, habits, and quirks. And he has traits born from the combination of theirs, traits neither of them have because Steven blends their personalities and gains new aspects from it.
Ruby is brash, short-tempered, combative, and always itching for a fight. She's devoted, and loyal, and she has a blazing passion for the interests she has, with an almost blind adoration of Sapphire.
Sapphire is calm, collected, semi-apathetic to her surroundings due to her foresight. She's as devoted to Ruby as Ruby is to her, and just as loyal, with an unshakeable confidence in the way of things.
Garnet, like Steven, is a blend of traits, and has her own born from the mix of them all.
Garnet is quick to action, but slow to anger (outside of very specific things), she's devoted and loyal to the utmost degree, she has a high self-confidence born from Ruby and Sapphire's love for each other, an unwavering belief that nothing that comes from their love could be bad, or wrong, and as she comes from that love she is good, and right.
Garnet is hard on herself when she makes mistakes in the moment, but quickly moves on when things are okay again, she learns from her mistakes and actively takes steps to avoid them again, a trait born from Ruby's combative mind and Sapphire's strategic mind.
"I am more than the two of them, everything they care about is what I am, I am their fury, I am their patience, I am a conversation."
She is not Ruby and Sapphire. She's not just a fused fighter, or a simple matter of 1+1 = 2. She's an entity all her own, someone else entirely, born from the love, and adoration, and fury, and frustration, the passion and the calm, the desperation and the relief, the joy and the sadness, the love and the hate...
She is a child, born of Ruby and Sapphire's decision to give up their physical forms in order to create something beautiful and new.
They would rather Garnet live, they would rather live as Garnet, in order to allow this new being, this person born from their love, to experience the universe and to know life and laughter and joy and love, than for them to do so apart.
Just like Steven's mother would rather Steven live, as Steven, in order for that new being, that person born from her love with Greg, to experience the universe and to know life and laughter and joy and love, than for her to do so herself.
Steven is an embodiment of everything Garnet stands for. Such utter devotion, and such true love, that one would give up their life for the sake of this new being.
The love of a parent, shown in the form of every single second this new being draws breath.
Just like Garnet feels the love between Ruby and Sapphire with every breath she draws of her own.
I think this is why Garnet and Steven are so close. They're one and the same, they understand each other. And where Steven is always learning, always growing, Garnet is too. She's been around far longer, but Steven brought a whole new perspective to the world. Finally, for the first time in her life, she knew someone just like her.
Someone who would never meet the parts of them that came together to create them.
Someone who understands what it means when someone gives up their life for yours.
Someone who, from birth, has known a form of love few others ever even hear of.
She finally had someone who could feel what she felt.
And Steven grew up with this woman, this person, who has never once doubted him, never once doubted what he could be.
Someone who understood that he was learning, and growing.
Someone who understood that he was new, and changing.
Someone who knew exactly how to show the form of love that he needed most.
Because how could she not?
How could she not adore this child, this boy who was so like her?
And how would her parents, how would Ruby and Sapphire, treat this child, this boy born from the same kind of love that carved Garnet into the universe?
And Garnet, bearing a heart stronger than twice theirs, did exactly that, from the first time she heard his tiny little voice, and forever on.
Because at the end of the day...