Unrequited Required
Point the First:
I would expand this question to "Can one ever really be friends with someone they find attractive?" because in this modern day and age it seems silly to leave out all possible combinations - can gay men be friends with each other? Can lesbians? It sounds as if the query is posed to suggest we are nothing but slaves to our hormones. If you want to be, go for it. Me? #!@% that shit.
Point the Second:
Growing up I struggled to keep many friends at all. I got sorted with the misfits mostly; and our gang was comprised of all types and sizes as there was always safety in numbers. I had many close friends who were girls. Many who were boys. When I played with girls I often played "girl" things - dolls, boy band fangirling, blah blah - and when I played with boys I often played "boy" things - Ghostbusters, TMNT, bam bam - without much difficulty making the shift. People who freak out when you suggest that gender is a result of social programming rather than a genetic imperative should probably have played more as kids.
Point the Third:
At any given point in a friendship I would argue there are reasons it never goes past a line. It stays in the friendzone - nobody puts you there, you choose it - because it takes two parties to complete a pass. If one side is unrequited, then it's your choice, settle or quit. If neither side feels it then no worries. Yet I wouldn't trade any of my friends for the thrill of a perhaps temporary flame. Same as I wouldn't avoid them if I suspected they might wish for more. You can set boundaries - after all, clarity is kindness - and leave it up to them. Again, it's a choice.
Conclusion: Of course men and women can be friends. The battle of the sexes is already too harsh at times to risk otherwise. If there can be no common or neutral ground, no recognition of the commonality between us and the stupidity of the lines drawn, then what hope do we have but to live in an endlessly divided society? What hope do our children have of ever hoping to achieve more than Dick & Jane?
Everything isn't always about sex, horndogs.
Grow up already :P