When a marriage is entered into by a man and a woman who do not withhold known information that would invalidate the marriage, the Church holds it to be valid. If however something is not disclosed the possibility of the marriage being null and void from the first moment exists.
Examples of non-disclosure would be, the husband or wife not disclosing they are homosexual, one or both of the parties not disclosing to the priest or each other that they intend to practice artificial birth control, not disclosing known infertility or impotence. Not disclosing sex change procedures. If someone entered into the marriage and did not know they where infertile, therefore did not enter the union attempting to deceive the grounds for annulment would not be upheld. The need to prove that someone with ambiguous sex would be important would be to protect the integrity of the sacrament and the souls of those who where attempting to marry. The reason being there would be questions with someone of ambiguous sex, whereas someone who did not have any evident anomalies would not have a reasonable question as to being fit to contract a marriage.