Why do guys say that girls think wrong when we make a joke, and then they act all pervy later?

In science a girl got mad at her seat partner and was like said, Drew, why are you so mean? I what did I do to you? My guy friend was being annoying, and said, "Everything!". After he said that I grinned and was like, "Whoa Chris, everything?" And he looked and me funny and told me I think really wrong. Later, in English, I told one of my friends that I'd make her a picture thing for her binder, and if she wanted it to look sideways on her binder, or if she wanted it going straight down. (Making it go straight down is harder.) She said straight down and moved her hands down parallel to make sure I understood. Her seat partner and my guy friend started mimcking her and laughing, and when i asked them why I was such a perv if they were laughing what she just did without realizing it, they were just like ,I don't know..

How come its such a big deal for girls to talk a little like that, and its nothing for a guy to act like a perv?