It’s our protagonist’s first day at a new school. She’s frazzled and distracted, her mind heavy with plot-relevant drama– and BAM! runs headfirst into someone, sending her books flying. She and this stranger look into each other’s eyes, and the readers are given a lavish description of how good-looking he is. Their hands brush as he passes her the books, and she immediately looks away, embarrassed by the contact. They make awkward smalltalk, and he welcomes her to the school and offers to show her to her classroom, adding in a joke that helps lighten the protagonist’s tension.
Oh– wait– I’m sorry, did you think he was her love interest? Goodness, no. He’s just a minor character! He’s just really nice, and she’s just socially awkward and easily embarrassed by physical contact. Why, what did you think was going on?
Sending mixed signals
There are certain tropes that your readers are naturally going to associate with romance. When they see two characters in that particular kind of situation or interacting in that particular kind of way, they’re usually going to assume that those two characters are intended love interests– whether or not you actually intend for those characters to share romantic feelings or subtext.
Often this happens by accident. Maybe the scene is supposed to show the characters’ platonic or familial bond, and it just went a little bit left. Maybe the chemistry they share is just that good. And maybe you were aiming for bromance and got romance instead.
It also happens intentionally, when the creator is trying to get a rise out of fans of a particular relationship, but isn’t actually willing to commit to that relationship. This happens most often in the form of Queerbaiting, where a character is strongly suggested to be homosexual or bisexual, only to have the bisexuality never confirmed and the homosexuality blatantly denied with a seemingly tacked-on significant other or an offhand denial.
Generally speaking, promising a relationship that you don’t intend to deliver is disrespectful of the fans, and should be avoided.
But they’re the same gender!
But they’re just friends!
Maybe they’re not. If two characters really do have that good of chemistry, consider actually making them love interests, even if they don’t end up together. After all, not all people find True Love ™ with their first love. If that’s not gonna work, go through the problem scenes with a fine-toothed comb and try to find the aspects that might be misconstrued as romantic. If you can’t find it, ask a friend to help. And please, for the love of puppies, don’t use the line “like a brother/sister to me”.
But they’re related!
As fans of Oedipus Rex, The Mortal Instruments, Star Wars, and Flowers in the Attic can attest, that doesn’t actually get in the way of romance nearly as often as you would think.
But one of them’s too young!
And eventually that one will turn eighteen (or whatever the legal age of consent is in your given area), and there will be much rejoicing. In this case, though, I would recommend explicitly stating the whole ‘not till you’re older’ thing– because otherwise you’ve got some unfortunate implications on your hands.
But one of them is asexual/aromantic!
Asexual people are still capable of having romantic relationships, and aromantic people are still capable of sexual ones. But if they are, I recommend mentioning it in your text or dialogue somewhere.