It is not contractual relationships themselves that anarchists take heed to, but rather the circumstances of such relations. We look to expand the arsenal of choices accessible to the people, as opposed to deciding among the available.
As an example, the owner of a slave could give the slave a choice to move rocks back and forth or endure whippings on the back; simply because there is a choice available does not entail the likes of a voluntary relationship. The slave picks to move rocks not because they would like to, but rather because it is merely preferable to the alternative.
When capitalists express their enthusiasm for contractual relationships, they do so in comparison to non-contractual-relationships, i.e. forced relationships. As anarchists wish to expand one's access to relations, an argument of these likes would equate to a misrepresentation fallacy, as well as a false dichotomy fallacy.
If, by chance, you are having trouble understanding my response, I would highly recommend BadMouseProduction's "But they have a choice!"; it explains this idea concisely.
@dot Am I allowed to use external links?