I do not think that being unable to raise your children on your own makes you unworthy of giving birth to and raising children.For context: The case involves a woman under the legal guardianship of her aunt, who is brain damaged and "cannot be left alone to operate a stove or perform most household chores." She has stated that she wants to have children someday, but the aunt has requested permission to consent to tubal ligation on her behalf.
It strikes me that being under the legal guardianship of another is a fairly bright line that distinguishes this case from some of the potentially abusive situations described by the commenters.
Update: I've been mulling this over. We do permit parents to consent to various medical procedures for children, even if those procedures are not medically necessary (some forms of cosmetic or reconstructive surgery, for example). However, we don't allow parents to sterilize their children because they are only guardians of the children until the latter reach the age of majority; the reproductive capacity is held in trust for the adults they are to become. For someone who is permanently childlike--someone, perhaps, who will never attain the mental maturity to provide legal consent to sex--what makes sterilization different? If it is, should long-term birth control likewise be prohibited? What other medications or procedures should a person under guardianship be permitted to decline?