I'm placing my bets right now:
Our-world protagonist is secretly a lesbian and has been in denial about it, even dating a guy she didn't like. Yuri-world protagonist was straight and couldn't accept a world where she couldn't date men. Yuri-world-protagonist found this alternate world where a lesbian version of herself was acting straight purely to stay "normal" and realized she could take her place, freeing Our-world-protagonist from feeling persecuted for loving women by giving her a reality where that is considered normal, while also instantly landing herself in an already-established relationship with a guy. Win/win. I also predict 1 of 3 endings:
1) Our-world-protagonist will find a way to get back to Our world, realize her boyfriend was happier with alternate her, realize alternate her was miserable in Yuri world, and realize she was happier in Yuri world than Our world, then swap back and just let their swapping-of-places last forever.
2) She'll swap their places back, which can't be undone, and she'll finally accept her lesbian-ness, stop worrying about being perceived as "normal," and respectfully break up with the boyfriend to date women in Our world, while the author will just gloss over the fact that Yuri-world her is stuck being miserable again.
3) She'll wake up and it will all be a dream or coma, wherein she discovered her "true feelings," freeing her to finally be herself in our world.