The story of Helen of Troy begins with the Judgement of Paris. Eris, the goddess of strife, angered at not being invited to the wedding of Peleus and Thetis tossed into the wedding party and apple inscribed "for the most beautiful". Paris, widely acknowledged the most handsome man alive, was asked to judge whether it should go to Hera, Athena, or Aphrodite. Hera offered Paris unlimited power, Athena offered him wisdom, and Aphrodite offered him Helen, the most beautiful woman in the world. Where the gods are concerned, the preferences of mortals and such minor inconveniences as prior marriages, don't really matter.
The Iliad doesn't state whether she went willingly or not, although most subsequent authors condemn her for wantonness, suggesting that she is traditionally viewed as having gone willingly. Since Aphrodite is the goddess of love, once Aphrodite had decreed that Paris would be awarded her, she really had no choice in the matter.