My copy of the poem (found in the Norton Anthology of Poetry) contains the subtitle "Annunciation." A footnote explains that Yeats wished to draw parallels between Mary's pregnancy by the Holy Spirit and Leda's rape by Zeus... an interesting thought.
William Butler Yeats' "Leda and the Swan" contain the diction of violation to create a theme of the violence and inevitability of human fate, using the story of Leda's rape.
There are several examples of the diction of violation. These are "sudden," "blow," "beating," "staggering," "terrified," "shudder," "broken," "brute," "blood," and "indifferent." Each of these words directly relates to the violence of rape. "Sudden" implies something swift and startling. "Blow" and "beating" are both descriptive of brutal treatment. "Staggering," "terrified," and "shudder," are all the reactions of a victim of rape; each connotes loathing, revulsion, and fear. "Brute" describes the aggressor and his actions; it is another word overflowing with primal violence. "Broken" and "blood" are both reminiscent of the damage done to a victim of a rape, both physically and emotionally. Finally, "indifferent" is a word very closely related to violation; the aggressor is callously uncaring towards the victim, as if she is not a living creature, but a mere tool for him to use.
Why would Yeats use the myth of Leda's rape by Zeus as a conceit for his theme, that human fate is violent and inevitable? The spawn of this rape was Helen of Troy, whose beauty ultimately would cause the fall of Troy and the ascension of Greece as a major power. All of this would happen through the terrible violence of war, and from the moment Zeus impregnated Leda it became inevitable. Throughout the poem, Zeus is a mighty figure, "the feathered glory." Leda, despite her fear, is unable to resist his advances; she has only "vague fingers" to vainly attempt to push him away. Here he represents fate, violently taking her, humanity. Yeats takes the myth and gives it greater portent; he even subtitles it "Annunciation," in reference to the Virgin's meeting with an angel to learn of her pregnancy. There, as in Leda's case, the course of human history was changed by a violent twist of fate. Here, myth becomes metaphor for the brutality of human existence.