It seems to me for personal reasons that the author ended the manga like this because he could have kept making more chapters but decided not to maybe the scope of the show became to big with its philosophies about humans and love but it could also be that the author just didn't want to draw or write golden boy anymore he has been from 1993 to 1998. But it is a shame that a series has to end like this on what feels like a cliff hanger but is just a really bad ending if you get what I mean.
(He also had history directing porn and AVs so that explains the heavy influence of sex)