Fusion happens for multiple different reasons, and it's more accurate to say it represents relationships in general, which includes but isn't entirely sex.
Standard fusions are done for strategic purposes, and at best embody the love soldiers feel for each other.
Stevonnie and Garnet are done for fully consumate love, and both of them are the most stable fusions.
Rainbow Quartz is definitely a very lustful fusion between two unusually lust-filled gems, which can be contrasted with the much more toned-down presentation of Garnet or Stevonnie.
Smoky Quartz was more of an embodiment of the sibling love between Steven and Amethyst.
Opal is like a tenuous, semi-stable friendship done for necessity.
Malachite is like the toxic relationships people enter into out of desperation.
Both Sardonyx and Sugilite represent the relationships that people enter in order to increase their social status and show off their hot "arm-candy"
If you want to compare it to sexual activity, go ahead, but it's not exactly the same, since it has little to do with reproduction i.e. the Kindergartens.Fusion is criminally underused as a concept in the show, I think. It'd be so cool to see Peridot and Lapis fuse with the CGs, and I don't think we'll ever actually see that