Sexual attraction is a funny thing, and sometimes if a character physically appears to be the right gender to appeal to their sexual preference, then they'll become attracted to them, regardless of what that character's actual gender might be. Usually said character's true gender is being kept secret purposefully and they are a male Disguised in Drag or a female disguised as a male, but whether the misunderstanding was intentional or not, due to Rule of Drama, they will find out and it will be awkward, hilarious and possibly traumatizing for all involved.