WARNING Rabbit Hole Ahead!

English gets stranger and stranger as one thinks about what one actually says. 

Okay, splain this:

Singular and plural stuff. One says a pair of... Which implicitly is a singular as indicated by the A proceeding pair. But then we say of geese, which is a plural. Or of boys which is a clear plural.

Properly, it seems to me that it should be a pair of boy.

Splain this without hand waving.