Yeah, pretty obvious that the girl she's having sex with in the intro isn't Saeko.
At this point in the story, she's still hesitant about sex.
In the intro, she's doing it like a rabbit.
So, we already know that their relationship is going to fail.
So as pointed out, the girl in the intro sure -talks- like Saeko, and acts like her, and while we don't see her face clearly, the hair length and style could easily be Saeko.
The fact that they're sexually active in the intro doesn't tell us anything. Even if the one-shot is predictive, we still know from the one-shot that Saeko and Miwa are sexually active and that Miwa really enjoys it.
(the rest of this is not directed at you specifically)
Honestly, I wish people would just shut up about how it ends. If a possible breakup ending is a dealbreaker, that's fine. Go away, comeback if you hear that it ended happily for the girls. If it's not, then chill out and stop fussing about it.
The fact is we don't know how it ends. We don't know if the author just wanted to expand on the characters while keeping the same result, or if they wanted to revisit them and see them as working out. We don't know if they're gonna break up the same way, or differently. We don't even know if the girls in the one-shot stay broken up. Exes get together all the time. They grow and become more compatible. Sometimes they grow apart.
There are innumerable good, well-told stories that could come out of this, so just...let the story be told, okay?
