So Saeko is wearing the triangle shorts the girl in the first chapter was wearing, so given the dialogue in that first chapter, I think that scene is the aftermath of what's happening here, and that the next chapter will bookend with that scene, bringing the first volume to a close.
Which means that the opening scene of the manga isn't at some nebulous time in the future, it's just after Miwa's first time with Saeko, which means it doesn't actually tell us anything about how their story ends. ;p
So yaaaaaay, we get to argue more about whether they stay together in the long run! ;p