As somebody who never watched the Cure franchise etc, is it canonically Rainbows everywhere or more
heterosexual life partners?
That is a complicated subject, there are a handful of Cures that the veneer of 'normal' that the Suits force them to wear is so thin and chipped that it is barely there
- In the Hugtto crossover movie, one of Nagisa's lines is a cultural touchstone lines in Japanese, referring to Honoka as being 'Her Most Important Person'. It's similar to the 'the Moon looks beautiful tonight' line from old samurai works. Where saying the two characters are dating is not allowed, but they can hint at it very firmly. (A non-romantic example of this same kind of thing would be Itachi from Naruto. When he tells Sasuke to live a long and unsightly life, that is a huge flag in Japanese literature that Itachi's motives were much more complicated than they seem.) Saki and Mai get some pretty heavy shades of it due to being... Well, serial number shaved copies of Nagisa and Honoka, but they don't ever get anything quite as blatant.
Then there is Fresh's Love Momozo and Setsuna. Technically Love has a love interest in the show, but there are like, three episodes dedicated to him during the latter half of the series where Love is so unaware of his interest that he basically ditches him to go and spend time alone with Setsuna instead. Poor confeses to Love to her face, and she completely fails to parse his interest.
There's the Suite Duo - there are two other cures who show up later in the season, but one of them is like 8, and the other is a cat. They have no love interests. But the tension between Hibiki and Kanade is so obvious that other characters make jokes about Hibiki being Kanade's wife, and if one of them is in a funk, their friend group's primary means of fixing it is 'Go find the other and throw them at them'. Their
parents make these jokes, and they never (other than one episode) actually argue against the jokes.
Dokidoki was mentioned, with the way ongoing joke that Mana is a harem protagonist. And honestly, given that she canonically marries
Kamen Rider Wizard that joke might be onto something.
Someone else mentioned Yukari and Akira from Kira Kira Precure A'la Mode... But they failed to mention the manga where Akira is kidnapped by a psychopathic BDSM witch who wants to melt Akira into chocolate fondue and bath in her, and through the entire thing Yukari is acting like a jilted lover... But Yukari and Akira's relationship is very heavily inspired by Uranus and Neptune...
Finally there is the Maho Duo - Who raise a child together (arguably two), and the Star Twinkle main duo - That series technically has five, but it has been flat out stated by the original writer that he wanted to write a whole series that would only have two cures, but since Maho was still very popular, Toei didn't want to compete against their own products, so he had to add in more cures. Something that becomes extremely obvious when the movie just puts the other three cures on a bus for the whole movie. But that series ends with a fifteen year time skip where Hikari, the main character,
invents faster than light travel for the sole purpose of meeting up with Lala again, and during that time both girls changed their hairstyles to the other's hairstyle in order to help them remember each other.
Admittedly, yes, part of it is just the suits at Toei making it so that they never really show it off on screen...
But there are plenty of cures who either don't have enough romantic chemistry with another cure to cut it with a knife, or who have decent amount of chemistry with male characters.
...Even when they honestly shouldn't.