I don't want to set the world on fire, I just want to say "yes" to my players and their ideas
I want to start by making an argument that might challenge pretty big-picture ideas about immersion in TTRPGS; Our players aren't obligated to feel like their identity and expression is limited to the canon of the setting.
Of course, as always, there's caveats to lay out all the way from the Commonwealth to the Hoover Dam, but we can and should be willing to adapt our version of canon to allow players to enjoy themselves. For starters, let's start by reminding ourselves that nearly always in TTRPGs *your story simply is not canon in any official capacity* and therefore we embrace that we're, in a way, breaking the agreed-upon reality of the story to tell our own.
That said, the Fallout setting and world has fantastic story elements that are otherwise limited in scope by nature. Factions rose and fell in rapid succession, creatures came out of laboratories locked in tight regions, and quite often you'll find it takes quite a lot of hand-waving to make your party of wanderers.
For example, super mutants are notably limited scope since the origin of the FEV experiment puts their creation to certain parts of the map of the United States, and Fallout games have generally had to expand the lore of the supermutants to explain their existence in other games. You can do the same for you supermutant players, and you more than likely will have to in order to make your own party's story line up. The first takeaway here is that you can and should add what you need to the story to make your beats line up.
But it goes further, since the phenotypical constraints of many of the playable origins in the Fallout 2d20 system are more limited than one might think. Canonically, supermutants are considered to be many things, but never "femme" and often (perhaps incorrectly) considered agender. I don't find this to work, and perhaps if it does I reject this idea at my table. There's much more to this topic than I have the capacity to cover, so I'll defer to you, the reader, to do your research and consult with your players about the wants and wishes of the party.
The role of the Game Master at a table should not be that of the gatekeeper or arbiter of personal expression, and one should feel free to expand gender and sexuality in PC and NPC representation to welcome diversity and acceptance beyond the typical norms of the given system and setting. The gendered supermutants, the nonbinary ghouls, the world as *you* see fit your *your* table is important.