I looked up the original because I was confused: this translation is all kind of wrongs.
The girl MC is a teacher (they call her "sensei" on the first bubble, but somehow, it's not mentioned here).
On page 2, it's starting a flashback : the boy is in primary school.
The one who got mad was the MC's mother, when THE MC introduced her lover to her own parents.
Implying it was probably a girl even then!
It's never mentioned she ever was with a guy, so the bisexual
tags stems from a bad translation.
The boy was just in an unrequited love from the start... no het here.
Blegh.
That would make more sense, yes. Especially with this bit:
Implying it was probably a girl even then!
I honestly got this feeling even from just this, just thought that she may have tried to hide it in the conversations, since the ending made it quite clear the boy knew he'd never had a chance due to this.