Giving someone their first cigarette is way worse than stealing a kiss, but Hyo-In isn’t that bad. She’s been considerate of Ah-Yeong’s feelings in the past, and her internal monologue is saying this isn’t like past flings, so I’m willing to say she wants a serious relationship here.
Narratively, we need something to trigger Ah-Yeong’s gay awakening. Joo-Hyun sure isn’t going to make the first move (if she even understands her own feelings), so somebody else has to put her in the mood to smooch girls. Hyo-In has drawn the short straw and is going to get her heart broken for it.