The novel opens with the dramatic aftermath of what happens when you have sex with your governess instead of your wife. Despite the reconciliation between the unfaithful Oblonsky and his wife Dolly, Oblonsky doesn't mend his ways. He continues being unfaithful, rationalizing that although his wife is no longer good-looking, his own good looks certainly shouldn't go to waste.
Levin adheres to essentially the opposite view. He's wracked with guilt because he isn't a virgin, and confesses his sexual history to his wife-to-be. "Fallen women" or "loose women" (with the notable exception of Anna) absolutely disgust him. This is a guy who digs chastity.
Then we've got Anna and Vronsky. For them, sex is a destructive force. Remember that scene where Vronsky's desire is finally "fulfilled"? Vronsky is described as a murderer and Anna is a murdered body. By sleeping with her, Vronsky annihilated in one fell swoop all of Anna's respectability and position in society.