It starts when we're young. Boys and girls are always teased about being in a relationship. You play with a boy at recess, and everyone's asking if you have a crush on him. All you did was play tag or go down a slide together, and it's already being transformed into something it's not. Then we move into middle school, and everyone starts getting into relationships (as much of a relationship as middle school dating can be). You're still friends with the boys, but you start to feel the pressure of having to "date" someone. You make it through middle school, and then you're in high school. That's when the relationships get real. Your classmates are getting into relationships to get married, and you're just trying to have fun. You may have a boyfriend or two, but you continue to be friends with the people you've known since kindergarten.
One of those friends, or maybe even more, happens to be a boy. You're in the same classes, you park a few spaces apart, and you consider him one of your really good friends. He has a girlfriend, and you've met her. You like her, and you support his relationship. And then you start to hear the rumors.
"I heard that she likes him." "Are you guys dating?" "Where's your new girlfriend?"
You can spend all the time you want denying them, but they'll never stop. Sure, we may joke around, and privately say that I'm his "side chick". But we know it's not true. We can say these things, because we know the true nature of our friendship.
And here's the thing: we don't have to define our relationship to you, because, frankly, it's none of your business. Have you ever stopped to consider that the age-old trend of teasing boys and girls who are friends about being in a relationship could hurt someone? I've had to listen to my friend complain about how he's worried that his girlfriend will be upset about our classmates teasing him about his "other girlfriend". I can't count the number of times I've said "WE'RE JUST FRIENDS". For some of you, you've never been able to have a relationship with someone of the opposite gender without it being full of flirting. If that's how you choose to formulate your relationships, that's fine, but I don't have to be the same way.
It seems that you may never be able to understand platonic boy-girl friendships while in high school, and honestly, I don't care. I will keep being friends with him, because I want to. And, honestly, it's none of your business.