My immediate reaction:
"I have a girlfriend already, but what am I supposed to do when another girl confesses to me?!"
Dude I'm all for Polyamory, but this entire line tells me you haven't discussed this with your current girlfriend, so the answer is simple. Tell them 'sorry but I'm in a relationship.'
- or break up with your current girlfriend, you don't seem like a healthy match tbh.

At no point after this do they actually have a discussion about boundaries and expectations in this relationship that they are undertaking. Also just moving in together, like even negating that they are teens. That's insane.

Also there is an absolute disregard to Saki's non-consent, but also the general manipulation that exists to allow for Naoya's ability to have his way. Along with the fact he never actually listened to her, she rejected him FOR YEARS. Like on top of this unhealthy relationship it promotes the idea that you can just wear someone down until they will date you.

The humor of this manga is placed in the fact that Saki is having feelings towards Nagisa. Making this the butt of a joke. Which is gross.

TLDR;
This is Toxic.
It's Queerbaiting and will cause whiplash!