I think what he's saying is that, as a 19 year old, any relationships he has now are doomed to fail after the ending of this game because he's young and relationships don't always last at that age.
That would seem cynical to me, even if there's some truth in it.
They will have endings and they won't be nonsensical. They will make sense within the context of this game and none of these relationships with the LIs are impossible so long as you make the right decisions.
You clearly just don't seem to understand the concept of a LI in these games.
Also, Josy never once said the word "boyfriend", she said she was "in a relationship" and the MC inferred it as "boyfriend" because he did not consider the possibility that she would be with a girl. And Josy is very much bi considering that she knows her way around a dick and Maya even asks her for advice on how to please a guy, so clearly she knows Josy has been with guys in the past.