Yeats' poem "Leda and the Swan" is probably based on an account in Ovid's Metamorphoses. In this account, it appears that Zeus seduces rather than rapes Leda, although given that Zeus is a god, one could not imagine human will resisting his desires easily. There is a sense of Leda being overwhelmed both by the superhuman intensity of Zeus' passion and the strangeness of the swan form. The rhetorical question of the octave seems to suggest the answer that it would have been impossible for Leda to even think about resisting him, much as one might not be able to imagine holding back a thunderstorm with one's hands. Zeus acts as a force of nature and necessity.
This sexual act sets in motion not only the birth of Helen and thus the Trojan War but also the great works of Homer and subsequent artists and writers. An important implication of the sestet is that without the tragic war we would not have the great artistic works (including the very poem we are reading) that the war engendered.
Although Leda is a pivotal figure in Greek mythology because this moment when Helen is conceived starts the events leading to the Trojan War and eventually to the death of Agamemnon, the narrator speculates as to whether Leda herself understood the significance of her actions or whether she was simply caught up in the intensity of the physical sensations. The poem does not answer the question of whether Leda gained some of Zeus' knowledge in this interaction or merely became pregnant with his child without understanding the significance of the event. Instead, the reader is left to think about the problem.
On a larger level, the example of this poem makes us as readers wonder about the degree to which we understand the significance of events in our own lives as they are happening or whether we can only understand the significance of events in retrospect.