I'm guessing the butterfly here represents the "butterfly effect". You know, a classic in quantum fiction.
And, since you guys brought up the possibility of Yuri!Marika being straight, I'm sharing my theories on the matter. In that universe, the last man on earth died roughly a century ago, most likely after a life of fucking non-stop in the vague, desperate hope of producing even a single son... lucky bastard.
Ehem Sorry, I got side tracked.
What I'm getting at here is that there still exists the concept of "man", even if they're all gone, and all they left is documents, paintings, Black and White photos, statues, coins, etc. And, much like we're fascinated by dinosaurs or ancient civilizations, I'd say that, if heterosexual women could exist in such a world, they'd have to resort to the aforementioned remains of man to, er, "satisfy their needs" (need a better way to say this). That could explain why Marika was called a pervert in the earlier chapter, even...
So yeah, I got an excuse to type that in. Go nuts I guess.