THANK YOU LESBIAN (or maybe Bisexual?) JESUS
There was a really interesting comment on the app (by jakan1234 if you're interested) in regards to ep.39. This person thinks that, based on SeolA's comment ("younger sister," my ass) and her troubled attitude, SeolA has had feelings for girls previously (probs in high school cuz flashbacks). She probably met a lot of guys in hopes of changing her prospects, but she was never really into any of them. While she was getting fed up with the endless dating chain, she met NoRae, who sparked those (gay) feelings of love she had when she was younger (which explains all the references to that random girl). And yeah, the commenter also thinks we'll be in for a long ride since they both assume that the other is hetero, and thus, try to hide their (gay) feelings.
I think this is the most plausible background I've heard so far, and I quite agree. SeolA is as gay as a little kid on Christmas morning, and NoRae is most likely bi.