I mean, it doesn't have to be a big sexual event like necessarily, but a meaningful cliffhanger is needed to conclude the part. Maybe a big fight or misunderstanding between the couple, or Kirito making a significant step forward into his corruption, which again would result in cracking the couple's relationship more, or Inoda taking full control of Kyoko.
But, yeah, irl sex is still just as possible as a conclusion as anything else. Basically, the same thing happened on the stormy night and in the treehouse; just a made-up situation that leads to it. Even if some of us don't like the pacing of it, there's nothing we can do about it, and we'll have to accept it anyway