The simplest possible answer is that it made her feel like property—and more specifically—a whore. Remember that this is an emotional response and she hasn’t had even twenty-four hours to digest this information. She already feels like her life has been stripped away and she is living like a prisoner. To be told that if the man she loves can’t do the deed, then they should just have someone else do it was demeaning.
Ali would have probably ended with the same conclusion Winslow had suggested, but it was too soon. Everyone was thinking it, but Gabriel and Joshua were at least kind enough to give her the space to figure it out. And Gabriel would never have allowed Ali to be forced, whereas Winslow, being a fearful man, would have considered force as an option. Ali sensed this and reacted to it. So besides feeling like control of her own body wasn’t hers to choose, she also felt unsafe.
On a more philosophical note, the value of an individual and the right to free will are both motifs in the series. There is a struggle between the two factions within the Concilium on whether the individual matters. Do the ends justify the means? Is it acceptable to take/strip individuals of their rights for the greater good?