Can't a guy and a girl be just FRIENDS?!?!

Why does everyone have to assume that I can't be just FRIENDS with a guy without wanting to date him or do anything with him?
For me, it's way easier to get along with guys because I was raised around 3 older brothers, so I know how to get along with them. I never had any sisters and to be honest I'm not one to be 'girly'. I just get along with guys better.

So one of my guy friends messaged me on facebook and asked if I wanted to hang out because he just bought some Supernatural dvds and he knows how much I love the show. I of course want to because I think it would be awesome to hang out with him and watch some Supernatural. I miss having people to talk to and I barely get to hang out with anyone anymore.
So, I ask my mom if I can hang out with him and her immediate reaction is no because he's a guy. I tried to explain to her that we would only be watching Supernatural and his whole family would be there. She then went on to say that she knows how this works and I couldn't go. She wouldn't even hear me out, at all.
I'm not five anymore, I'm sixteen - almost seventeen. I know some teenagers do fool around and stuff, but I'm not like that and neither is he. We were literally only going to watch Supernatural! It's not like my parents wouldn't be able to meet him and his parents before I hung out with him.

Every single person I have talked to, says that I can't be just friends with him. The truth is, I CAN be just FRIENDS with him. I don't want anything more and neither does he. We just get along because we know what each of us are going through.
Why do people think that a guy and a girl can't be just friends without it meaning something more? It's not right. I just want to be able to hang with all my friends, even if they are guys.