"Lullaby" is a love poem, so it's not so surprising that Venus would show up in it. Venus is the Roman goddess of love; her Greek counterpart is Aphrodite (you might have heard of her, too). Poets and artists have been a little obsessed with her for centuries; she symbolizes love, sex, desire, beauty, and all that other good stuff. In "Lullaby," she's a (mostly) benevolent figure who bestows love and hope upon the lovers.
- Lines 11-17: The speaker imagines two lovers all entangled. Venus looks down on them, and sends them a "grave vision." The word "grave" connotes seriousness and maybe even mortality, but in this vision, we get "supernatural sympathy" and "universal love and hope." So, we've got the typical Auden thing going on here: he mixes love and beauty with darkness and death. The speaker is pretty resolute in his lack of belief in heavenly powers, but Venus is more of a literary character than she is a god. (Not many Americans really believe in the Roman or Greek gods these days; they're usually much more symbolic).