The internet is set up to be genderless. In this manner, we are equals. Guys won't jockey to win a lady's favor, misogynists don't have a ready target, and guys don't feel like they've walked into the shower room and discovered that one of their classmates has different plumbing.
So, if I treat you like one of the guys, don't be offended. It means I view you as a person, and not a sex object.