There are earlier yuri manga you can look at that reference same-sex marriage being legal in various European countries, but it's always pointed out kind of wistfully, that some random country far away is more accepting, there's no realistic plan to go there.
But the US isn't just a military superpower, it's a cultural one as well, especially in East Asia. Students study English, pop musicians include English lyrics because they're cool and they export well across the region, American movies and TV shows are watched and enjoyed. America is both cool and familiar.
And really you can look at the evolution of the way that yuri manga talks about sexual identity as tracking a great deal with the growth of gay civil rights in America. Classic yuri often has that star-crossed lovers vibe, that their love is beautiful but inherently tragic, or it goes the other way and just treats it as a childish fancy that will be grown out of eventually. But now magazines like Yuri Hime have stories that are increasingly confident in viewing their characters sexuality as an identity, that they are writing lesbian love stories, and their love is equally valid as a het romance. Even the Japanese term for lesbian is an English loanword.
Asia paid attention to the growing visibility of gay people in the US and the dialogues in the US about gay identity during the gay marriage debate, and when gay marriage was legalized nationwide they really noticed. That's not just some far-away country in Europe they don't know anything about, they're familiar with America! They consume the culture, they travel there, they send the kids to college there. I don't have the link to hand but even in Korea, which is more socially conservative than Japan, the press was interviewing a Korean gay rights activist and he said how important the US legalizing gay marriage was to his own movement. He said that Koreans want to be like the US because the US is cool, so legalizing gay marriage would send a big signal that maybe it's actually OK, especially to younger people.