WARNING: The following contains spoilers for Sailor Moon Eternal: The Movie, now available on Netflix.
Sailor Moon Eternal: The Movie is streaming on Netflix, which adapts the entire Dream arc of Naoko Takeuchi's Pretty Guardian Sailor Moon manga in two feature-length films. New villains arrive on the scene and the Sailor Guardians return to take them on, including the Outer Guardians first introduced in Sailor Moon Crystal Season 3. In addition to preparing for battle against Queen Nehelenia and the Dead Moon Circus, the movie also gives a glimpse into what their lives have been like since defeating Master Pharoah 90 in Season 3.
Sailors Uranus, Neptune, Pluto and Saturn are the Outer Guardians, who protect the solar system from alien threats. Without any aliens threatening the Earth in the six months following their first battle, the Outer Guardians simply carried on with their usual lives. Uranus (Haruka Tenoh) participated in a car race in Africa, Neptune (Michiru Kaioh) held a violin concert in Vienna and Pluto (Setsuna Meioh) continued working as a scientific researcher in the field of physics.
In between their day jobs, the Outers also committed to raising their youngest member -- baby Saturn (Hotaru Tomoe) -- which they symbolized by wearing promise rings. Though the three older Outer Guardians reasoned this was all it was, for Haruka and Michiru, raising baby Hotaru may not be all there is to it. After all, they were already a couple at the start of Crystal Season 3.
Little is known about how Haruka and Michiru met, but it was established early on that they were a well-known romantic couple in Japan. Haruka -- who is non-binary -- was often thought of as "Michiru's boyfriend" which they never denied. Very early on in Crystal, they were shown to always go places together, and when alone in their apartment, they were shown to be quite intimate with each other. Fast forward to Eternal, and the couple are shown to have taken their relationship to the next level.
One of the first things residents of Tokyo's Juban district observe is that Haruka and Michiru have been wearing matching rings for six months. It's immediately assumed that they married at some point, which their public display of affection seems to confirm while they're out shopping. On top of that, Haruka at one point communicates to Michiru that diapers and other baby supplies are on sale, prompting Michiru to remind Haruka that their "little princess had already graduated from those a long while ago." This alone shows that they've started behaving more like spouses, which is further supported by their domestic life.
Haruka and Michiru live together in a small manor with Setsuna, who co-parents Hotaru with them. When Hotaru talks to her new legal guardians, she always refers to them as "Haruka-papa," "Michiru-mama" and "Setsuna-mama," indicating the roles they've taken on. In practice, Haruka and Michiru do take on the more traditional father and mother roles, with Haruka functioning as "the cool dad" Hotaru likes spending time with.
Though they may only admit the promise rings symbolize their commitment to raising Hotaru, it's also pretty clear through their actions that the rings symbolize much more than that. For the three older Outer Guardians, the rings symbolize a commitment to live together as a family. For Haruka and Michiru, it's also a commitment to each other as a romantic couple, indicating how much their relationship has evolved.