As Pibi-nim said, Eunsan is still hetero at the end of S1. And I think it makes sense, he is still figuring out Ian and his motive. They both have unknown history together (will be developed in S2) and discovering yourself can be long or challenging, because you have to admit something you never would've suspected about yourself, up until it was brought up. Moreover navigating your identity and sexuality shouldn't be decided on whim without second thoughts. Eunsan had physical activities with women and men, so it explains why he could accept the '4-month fling with no strings' attached with Ian, he isn't ignorant and is ok to get physical regardless of gender. However, he only had emotional bonds with women and never considered men, meaning that it's plausible for Eunsan to still be straight as of now, because we still have yet to see his introspection on cosindering romantic attraction toward men.