William Butler Yeats' "Leda and the Swan"

This poem was extremely confusing to me.  At first, I thought that when the poem started out by saying "A sudden blow:", that the writer was talking about shooting the swan.  This idea was reinforced again in the last stanza when the poem reads "So mastered by the brute blood of the air."  I am not really sure about this poem.  However, after re-reading it a numerous amount of times, I googled the poem and to see if there was any website that could help me understand it more.  A couple websites talked about how the swan is Zeus and he has come down to earth from the above skies and that Leda was raped by Zeus and now is going to lay Zeus' eggs.  There is no way I could have interpreted this from just reading the poem itself.  I do not really know whether I like this poem or not.