I only play Isabella's route. So if Isabella finds a boyfriend if she wasn't chosen by the MC, it won't be a problem for me (since I'll never see that scenario).
However, most players are completionists. I believe they will find it "weird" to see their favorite girl with someone else, while they play other girls' routes to unlock the gallery.
It sounds absurd, but it's not. This thing of "forcing relationships so other girls won't be alone" is something delicate. Fans would create a civil war if the DPC added MMF with John Boy and Elena, and it doesn't even make sense for fans to revolt that.
And this opinion comes from a guy who has the nickname "saynotoNTR"! Seriously, players are very sensitive. It's better to give up realism and leave all the girls alone.