According to "Pastor's Corner" at sspx.org:

4 We know of husbands and fathers whose wife or children have made a religious commitment in the Opus Dei without their knowledge. This constitutes an infringement against natural law.

Now whether or not Opus Dei is a good organization or not has nothing to do with whether or not it's an "infringement of the natural law" to make a religious commitment without the knowledge of parents.

St. Alphonsus Liguori:

Means to Preserve a Religious Vocation
Circuмstances sometimes prevent one who is called to religious life from
immediately following his vocation. In such a case he ought to be exceedingly careful
to guard so precious a Jєωel. This he can do best by secrecy, prayer and recollection.
As a rule it is prudent to keep one's vocation secret from everybody except one's
spiritual Father. Others would be apt to say that he could serve God in the world as
well if not better than in religious life. Without doubt, one who is not called to
religious life may serve God in every place; but one who is called, and prefers to
remain in the world, will as I have said above find it extremely difficult to serve God
and lead a good life.

Nor is it necessary to mention the matter to parents, unless one is certain that one
 has nothing but encouragement to expect from them. In this respect, all things being
equal, children are not bound to obey their parents when they seek to prevent them
from obeying the call of God. The Council of Toledo says: "It shall be lawful for
children to take upon themselves the yoke of religious observance, whether it be
with the consent of their parents, or only the wish of their own hearts." (1) The
same is prescribed by the Council of Tribur, and taught by St. Ambrose, St. Jerome,
St. Augustine, St. Bernard, St. Thomas and St. John Chrysostom. The last mentioned
saint says: "When parents stand in the way in spiritual things, they ought not even
to be recognized." (2) St. Thomas says: "Servants are not bound to obey their
masters, nor children their parents with regard to contracting matrimony,
preserving virginity and such like things." (3) In his treatise on religious vocation,
Father Penamonti thinks with Sanchez, Commenchio and others, that a child when
contemplating marriage is bound to take counsel of his parents because in such
matters they have more experience than the young. But, with regard to religious
vocation, he says, a child is not bound to take counsel of his parents, because in this
matter they have had no experience, and not unfrequently they are hostile to the
wishes or intention of the child, St. Thomas says: "Frequently our friends according
to the flesh are opposed to our spiritual good."

If one wishes to criticize the dangerous beliefs and practices of Opus Dei, and establish that wives and daughters should not make commitments to  the organization, that is quite a bit different than asserting it's a violation of the natural law for religious commitments to be made without knowledge of the husband or father.  

I don't know what parent would NOT WANT their child to become a religious. That, to me, is just unbelievably sad.

I don't know what parent would NOT WANT their child to become a religious. That, to me, is just unbelievably sad.

I am sort of in this category. It's not that I would NOT want my child to be a religious, but I'll be honest... If he does have a vocation, I would be much more relieved as a mother if he is called to be a humble, barely noticed, maybe even underestimated, little porter out at some monastery somewhere than if he is called to a more public and pressured position of priesthood. I have girl friends who have "rose-colored glasses" over visions of sons as priests and while I understand the sentiment and know we need dreams of magnanimity... from the protective mother's perspective of wanting him to have the straightest, sweetest journey to heaven possible, the humble porter is the way to go.

But I keep that to myself and won't ever mention it to him because I know it's a bit selfish. There are a few signs (like my son having delays and learning disabilities that would probably prevent priesthood) that show that God might humor my mother's heart. He has humored me in a very special way once before. But we'll see. He may marry, he may live at home forever, he may indeed become a priest, we have no idea what to expect.  

The vows of Opus Dei are not only for their religious. They make their third order take vows. Which include not telling people they are in Opus Dei.

Opus Dei is a cult. They allowed non-catholics into their organization before V2.
They are grossly masonic.

There is a great article on Opus Dei. I'll try and find it.

http://www.sspx.org/miscellaneous/opus_dei.htm

No doubt Opus Dei is bad.  

The problem is the sweeping statement that it's a violation of natural law to not inform parents of a religious commitment.

Hasn't it been the case that many virgins in the history of the Church have kept their vows secret from their families?

http://en.wikipedia.org/wiki/Pharaildis

Not that I think it would do much good,

I am certain of that.

 

but have you contacted the people responsible for the erroneous comment?

If it is who I suspect then I doubt I would have any more success than before.