So I guess Steven and Connie? And Steven and Amethyst? Plus Garnet's just fucking 24/7? Or the 5 Ruby orgy?
When you start looking at the bigger scope of fusion, especially its purpose on Homeworld vs what it's become on Earth, the sex allusion by itself falls apart. It works better as an allusion for relationships; Homeworld's sense of it being a work relationship, but on Earth it's become about family, friends, or romantic (although also work sometimes, such as Opal). There's a wider breadth to this than "it's about sex." I didn't say anything before because it was Fade being Fade, but I think you guys can appreciate that there's a lot more going on here, especially when there are so many more examples where that idea doesn't fit.
That is the thing though, relationships, when it comes to being physical, are about sex. There are emotional ties, obviously, but when it comes down to it, old or young, sex is a big part of what holds the
physical side of a relationship together. You can't get more physical then fusion.
Yes, you can have platonic relationships, sometimes those platonic relationships still may have sexual desires or interests in each other for reasons of recreation, or need, or desperation. Sex isn't just about romance either.
Steven and Amethyst's fusion came at a time they both felt vulnerable, mostly Amethyst, who lost her confidence in herself and her abilities. Steven helped bring her back from that, and they immediately fused on touching. It's a common trope that the down-trodden person finds reassurance of her self-confidence through sex, as Pearl did through Garnet. Also yes, Garnet is literally fucking 24/7 because Ruby and Sapphire are that type of couple that can't keep themselves off each other (as they showed to the groups determent a few episodes ago during the baseball game), they are literally a walking PDA who want to get into each others pants at every turn.
Connie was introduced to the show not as "just a friend" but someone Steven openly, and romantically, pines over. Them forming into Stevonnie is their way of, quite literally, growing up, much in the same way many consider sex a rite of passage, it's what allows them to be adults (even if just for a little bit). Also, before anyone goes "They are kids!", don't forget this show was made by Rebecca Suger, a person who has drawn gay Ed, Edd, and Eddy porn, this isn't exactly new for her.
The Rubies actually do still fit in line with the allusion. Rubies don't choose who they fuse with, no Gem does, as the Diamonds dictate the purpose of fusion and finds Gems outside their quality and make that fuse to be "perversions" (which throws other allusions I am sure are obvious). Fusion, to them, is nothing more then a function. You ever heard the old saying, "Everything in the world is about sex except sex. Sex is about power". Homeworld takes that to the extreme, fusion is about power, but like the super space prudes they are, the natural way to fuse is only between the same race and only for the purposes of supporting the gem race, not for the purposes of love. Ruby made it clear that when Rubies fuse, they retain their own minds, they don't mingle. They just do it because they have to do it, and it's pretty obvious to me Rebecca is using them to allude to issues she believes exist in the world when it comes to sex and detachment, and social norms.
Remember the episode when Greg met Rose? Remember how Pearl had a mic-drop moment where she fused with Rose right in front of Greg and rubbed it in his face? How Greg himself wanted to fuse with her throughout the rest of the episode and how self-conscious he felt because he knew he biologically couldn't give her what she needed? Are you telling me none of that physical display represented any type of sex or display of power and ownership through sex?