I have theorized in SU, there are different kinds of fusion relationships.
Platonic- the kind of relationship friends have. For example, Garnet, Pearl, and Amethyst have a platonic relationship with each other. But since their personalitys clash and they can't balance it out, the fusions are monsterous like Sugilite and Alexandrite.
Maternally- the kind of relationship parents have with their children. I think this represents Pearl and Amethyst exclusively. As seen in story for Steven, Pearl treats Amy like a mother would would with her daughter. The reason they don't act like that now is that Amy most likely grew mentally into a teeenish form and is now starting to rebel like a teen would. Even though their personlitys clash, they still have that relationship, which results in a moderately looking Opal. Steven and the CG also have that kind of relationship too, but their fusion is yet to be seen.
Romantic- the kind of relationship lovers have.
Ruby and Sapphire have that relationship. But they take it to the next level. For fear of being separated, they fuse toghther forever. Because they love each other so much, Garnet looks completely normal, except for her three eyes. You can also see this in Stevonnie. Because Connie and Steven are in love, they get normal fusion
Anyways, thanks for reading, bye!