The See of Rome hasn’t made a direct pronouncement on the matter and Catholic theologians and philosophers are divided about it.
news
washingtonpost.com/wp-dyn/content/article/2005/05/30/AR2005053000872.html
zenit.org/english/visualizza.phtml?sid=13792
from the first link:
*It is a birthright that places him, and at least 80 other children born in a similar manner, in the middle of the boisterous political battle over stem cell research and a sharpening theological debate, particularly within the Roman Catholic Church. Some Catholic theologians are encouraging married couples to adopt unwanted embryos from fertility clinics. Others vehemently oppose the idea, calling it a grave violation of the principle that procreation should occur naturally.
The Vatican has not yet taken a stand. But if Pope Benedict XVI rules against embryo adoption, as some doctrinal conservatives expect, it could create a fissure between Catholics and evangelical Protestants, who have enthusiastically promoted embryo adoption and enlisted the White House’s support for it.*
commentary
humanlifereview.com/2001_summer/caulfield_s2001.php
losangelesmission.com//ed/articles/1999/0599jmc.htm
nationalcatholicreporter.org/word/word031105.htm
From the last link:
*Pacholczyk argued that the implantation of an embryo is an intrinsically evil act, because it separates aspects of procreation that should remain united. Procreation, understood as “collaboration between husband, wife and almighty God,” he argued, extends from fertilization through conjugal relations to birth, and to skip one of those stages means instrumentalizing the process.
It also results, he said, in a “fissure in parenthood,” since “spouses have an exclusive right to become mother and father solely through each other.”
“One should not become a parent through any means other than one’s spouse,” Pacholczyk said. He also argued that since fathers are incidental to the process of embryo adoption, fatherhood “is gravely and intrinsically violated.”
If Pacholczyk began from the point of view of the marital relationship, Williams began from the embryo.*
I think it it’s part of the marriage vow to only have children with one’s spouse. By engaging in “embryo adoption” one is having children with someone other than one’s spouse.