How is every binary relation BCNF?
A relation is in BCNF when, for all FDs that apply to the relation, the left hand side is a superkey. That is, it has to contain all the attributes of at least one key. Your relation must have at least one candidate key (that is, you can't have repeating rows in the relation), else it wouldn't even meet the conditions for 1NF. 

Given this, your relation can have zero, one or two FDs. The case for zero FDs is trivial (ab->ab). For one FD (a->b) the left hand side attribute is clearly the key.

In the case where you have FDs in both directions (a->b and b->a) you simply have two different keys, and so the left hand side of each FD remains a superkey.