Because in the Fallout Universe they seem to have settled most modern bigotries well before the bombs fell. I don't know if they even had MLK. Sure, they had a big problem with sinophobia, but that makes sense given that A) They were at war with China, and B) it was an oppressive police state in dire need of some ‘other’ to hate and blame.
No, a non-binary character in Fallout isn't some lore breaker. Them being a part of the BoS, a group infamous for not getting on with non-heteronormative folk, is.
(I mean, Veronica Santangelo's orientation already made her pretty unpopular with the Mojave Chapter, even before her questions got her relegated to boring-ass milk runs.)
But no, LGBTQ+ people in Fallout is not odd or unprecedented. The aforementioned Veronica, Gannon Arcade, a possibly-trans super mutant in the first game, same-sex marriage in the second, a lesbian couple and their adopted son and a gay couple selling weapons in the third, being able to be gay, lesbian, straight, or full on fucking bisexual in New Vegas, being able to sleep with almost any companion regardless of gender in 4 - this shit has roots, son. Stop acting like your imaginary ‘wokie’ enemies have infiltrated your fandom.