The heart wants what it can't have...
Regardless yes of course they can be friends, the question is are they actually your "friend"?
For me the issue with this subject comes down to honesty and communication. As has been shown and stated by many there is a lot of using and abusing going on. The reason its called the "friend zone" is because they are basically using friendship as a way to manipulate someone. A real friend will be open and honest with you and tell you how they feel. Most women do not do this and instead use their "friends" kindness since its basically free resources. My own mother admitted to doing this when she was younger.
To be my friend I require honesty and openness, regardless of gender. As far as sex goes, who said that sex and friendship are mutually exclusive? You can be friends AND have sex AND it can be a non romantic relationship. For me and it seems like many others share this view, A romantic relationship is more about having that special someone who you can share everything with, for lack of a better term a life companion. The friendship part is still there but they are basically the person you are closest too, your best friend.
Some other things to note, the keyword in boy/girlfriend is friend. If any of you have tried online dating, specifically the men, I'm sure you've noticed the surprisingly large number of women who say they want "friends first". In other words they want to use friendship as a form of rejection so they don't have to feel bad about outright rejecting you. IE the "friend zone". The main issue always has been and always will be a lack of honesty and communication. Manipulating language (the word "friend" in particular) and not being clear about ones intentions are usually where men and women have issues with friendship.
Sadly there's still a lot of women who use men by leading them on and manipulating the word "friend" to mean whatever is convenient at the time.
Not to let guys off the hook there are still guys who are not open about wanting to have a sexual relationship with a women and instead try to hide it only to come out later and say "Surprise! I have a penis!"
Honesty and openness are key to any real relationship be it romantic or platonic.