When Nymphadora Tonks first realized she was a Metamorphagus, she was questioning her gender.
Looking at herself in the mirror, she sighed, wishing that the notably female parts of her body didn't exist. The next moment, they didn't.
She screamed. She would deny this later, but she did. After a second, though, she realized she had made her anatomy male. Thinking that this wasn't quite right either, she closed her eyes tight and concentrated really hard on what she wanted her body to look like.
Tentatively opening her eyes, she looked upon her reflection with delight.
It was perfect. Not fully male, but also not fully female. But female enough to fit her view of herself as somewhat female. She would've described herself as a genderfluid demigirl if she'd had the right words.
As the years went on, she would alter her body to fit her current anatomical wish, but not altering her assessment of herself as mostly female. Sometimes she wanted a more masculine anatomy, sometimes more female. Occasionally she would want a 100% neutral anatomy. A few times she even experimented with a wholly male or female body, but she would invariably crinkle her nose in disgust and switch back to somewhere in the middle.
Just after Teddy was born, Remus caught her changing her anatomy. When she defiantly explained what she was doing and why, he nodded thoughtfully. "Good thing you're a Metamorphagus, then," he said. Then he turned back to bouncing their son. "Isn't Mommy resourceful? Someday, you'll be as clever as her," he cooed at little Teddy, who gurgled happily at the both of them.
That was it. No judgment, no weirdness, just acceptance. She fell in love with Remus just a little bit more in that moment.
After their deaths, they watched young Teddy grow up. They watched him take his first steps, say his first words, and cast his first spell.
Then one day, they watched him alter his anatomy. Just like his mother had. Or rather, their mother. Teddy changed so that their anatomy matched their identity. They couldn't explain how, but they felt their parents' love and acceptance, even from beyond. They would later wonder if they had just imagined it, but it was that moment that they used to conjure their Patronus.
Tonks shook with anger when Rita Skeeter misgendered them in her article reporting on the Quidditch World Cup in 2014, despite Remus reminding her that Teddy wasn't out to anyone outside of the DA families.
They both cheered when Teddy found love with Victoire, and when Molly Weasley made special effort to make sure that, at least in her house, people would be respectful of Teddy's gender. Anyone who misgendered them had to degnome the garden. She caught herself once and dutifully set out for the garden.
Both of the Metamorphagi in the Tonks family line found love, acceptance, and respect from everyone whose opinion was worth two Knuts to them. All was well.