Whenever you're talking about the Greek gods, you can be sure that the subject of fate and free will lurks somewhere in the background. Or in the foreground, as in this poem. Leda's rape quite literally changes the fate of the world: it leads to a catastrophic war. But the war wasn't entirely a bad thing, because, according to mythology, it led to the rise of European civilization. Unfortunately, Leda didn't have any choice in whether she would become a major world-historical figure. She was too confused and terrified to stop the rape. To further complicate matters, Yeats suggests that Leda might have had ambivalent feelings about the rape as it was taking place. Did she know how her own story would end? Did she welcome the knowledge?
We know that the speaker belongs to the modern age because he tries to question Leda's inner thoughts and will. For the Greeks, such speculation would have been irrelevant.