I think she feels "disconnected" from her peers in Seoul, especially when they talk about their parents, she can't really relate with their relationship with their family and is also jealous of said relationship (even if she hates feeling that way). This inability to truly and connect relate with her friends in Seoul made her feel isolated and not "normal". So for her being "normal" is really important and if you're not, you become isolated which is very painful (this is her point of view). She seems to see Haebeom a bit like a older brother, or at least someone she really cares about (in a familial way I think, not a romantic one), so him and Ho being in a relationship when she knows they can both be with a woman, basically means to her that they CHOOSE not to be "normal", which is just unbelievable and, well, kinda unacceptable to her who so want to be "normal", with a "normal" family, but just cannot be. To her credit, it's true that homosexuality and especially having a child raised by an homosexual couple is far less socially accepted than in the US or western Europe.
I meant "far less accepted in Korea than..." in my last sentence, sorry.
I thought she would have a better reason for her to be against them. Her reason is just sloppy writing, it’s only there to drive the plot forward. I’d be satisfied if she was homophobic, had a boyfriend that cheated on her with another guy, or had a traditional mindset- a relationship should be between a man and a woman. Maybe it’s not clicking for me, but what does her being neglected have to do with interfering in their relationship?