Why do most people think we are a gay couple?

well, I'm 17 and so is my best friend, we have been best friends for 6 years now, we share clothes , food, bags... anything and we are what you may say inseparable, I do adore her a lot, when we are together we hold hand, we hug, we kiss each cheek all the time, she come over to my house like 5 times a week and she would never lie or hide anything from me but lately people started to glare at us in very weird way, yesterday, I found the word " " written in pink on my locker and on my seat as well . it's getting really annoying I'm straight and I don't usually go around holding or kissing girls, my best friend is a special case . What should I do ? it's really frustrating . I'm concerned about my best friend's feelings because people never stop teasing her and spreading nasty rumors about her all over the school.