A long time ago when I was working on this, I saw a ton of developers in these indie projects try to do multiple love routes and end up abandoning their game because it's so much work. The other problem with multiple routes is that you need a lot more people to work on the project with you to complete it in a timely fashion. You end up doing regular updates but people are only interested in one character so they get an update with their interest in it then have to wait another year before that character comes back around.
Straight!? only has one route. It's more the story of Zack and his relationship with others than it is a dating sim. There are two routes in the story, which is what you're talking about, but those basically change the tone of the story moreso than being an entirely different story choice. Since I'd never done any of this before, I elected to do one "love interest" and do it right versus having like 5 really shallow choices. But imo, if there's a love interest in this story, it's Zack trying to love himself.