...sex is a river of fire that must be banked and cooled by a hundred restraints if it is not to consume in chaos both the individual and the group.

if it(the fire) is not to comsume... 这是be to do结构用于真实条件句，对将来可能发生的条件作假设。肯定句表示如果想要某事在将来发生，否定句表示如果想要某事在将来不发生。

If we are to get there in time, we'll have to hurry up. 我们想要及时到达那里，就得赶快。

网友句子中if从句意为，如果不想让欲火耗尽..., 就必须...。