Everyone relax. It's just Heath's curiosity, and knowing this manga with problems like these: They get resolved quickly and peacefully without much crippling misunderstandings
I took a glance at the raws and it looks like this comes up two chapters from now. Heath tells Lily what happened casually. Lily gets upset and and Heath can't understand why until Lily has Heath imagine Lily kissing someone else. Heath immediately hates the thought and they talk it through before the chapter ends happily.