Ugh, they used that troupe again?!

Well, maybe not... I'll admit I have not seen this movie yet, that's why I'm on here asking this question. I was intrigued when I heard this movie had two girls in a relationship with each other, but then when I read the storyline on here about how one of the girls ends up having sex with a male teacher and he gets her pregnant I thought; oh come on! Not again! Even in Korean cinema they have that ridiculous and offensive "lesbian has sex with a man" troupe!!! You hardly ever see it being the other way around. Couldn't the story have been just as heartbreaking if she was just cheating on her with another female? It would still be hard for the other girl knowing that her lover was unfaithful to her, right? However, my question is, do they make it clear what the girl's sexual orientation is? I mean, if she was really bisexual and just happened to be in love with the other girl, then I guess it's okay. So was she really bi? Or perhaps they are straight, and they just fell in love with each other? I'm really curious to know, because I'm still thinking about watching this.