Can there ever be pure platonic relationships? (spoilers)

I was liking Zach and Sam in that it seemed in the beginning, their friendship was platonic. That Sam was supporting his relationship with Amy. But then, we find out that Sam had a crush on Zach. Why does it always have to be where a guy and a girl cannot be friends without one of them crushing on the other or if that doesn't happen, it means that one of them is gay?