The Church is opposed to IVF in all circumstances. The answer to this is found in John Paul's theology of the body and other church documents. In sum, bearing children is thought of as primarily a natural result of sex between a man and a woman. They are not objects to be sought out as assets, but rather persons that come about via a relationship sanctioned by God. They are to be accepted as freely given gifts, not via cash transactions.

The Church does not value having as many children as possible. In fact, the Church encourages all its adherents to remain celibate (as St Paul says this is preferable to marriage), but insofar as many Catholics are called to marriage, rather than celibacy, those couples are to accept children if God so desires it. And if God does not desire it, then they are to accept that state too.

IVF reduces children and people to commodities to be bought and sold under what amounts to contract law. This is not the way that children should be treated (again, according to the church, the creation of a child is to be thought of as a gift from God, not a thing to be sought out).

When done in the context of a married couple, IVF separates sex from procreation, which is a sin akin to masturbation.

When done in the context of a single person, IVF separates sex from procreation, as well as intentionally deprives a child of their natural right to know and be raised by both their natural mother and father (this is also encapsulated in the UN declaration of universal human rights). The key word here is intentionally. Obviously, other forms of child rearing such as adoption or fostering also often involve a child being deprived of their right to know their biological mother or father, but these are not due to any sin of the adopting parents (hopefully... obviously, sometimes adoption is done maliciously).

From a practical perspective, IVF also involves the intentional destruction of embryos, either via culling or selective implantation and destruction. However, this is the sin of abortion. Creating the embryos themselves was also a sin.

The Church makes its teachings clear in a document published by the Congregation for the Doctrine of Faith, Dignitas Personae [1]:

> With regard to the treatment of infertility, new medical techniques must respect three fundamental goods: a) the right to life and to physical integrity of every human being from conception to natural death; b) the unity of marriage, which means reciprocal respect for the right within marriage to become a father or mother only together with the other spouse;[19] c) the specifically human values of sexuality which require “that the procreation of a human person be brought about as the fruit of the conjugal act specific to the love between spouses”. (part ii, para 12)

It goes on to state the difference between IVF and other fertility aids (for example, Viagra or Clomid, which are acceptable under appropriate medical supervision):

> In light of this principle, all techniques of heterologous artificial fertilization,[22] as well as those techniques of homologous artificial fertilization[23] which substitute for the conjugal act, are to be excluded. On the other hand, techniques which act as an aid to the conjugal act and its fertility are permitted.

Note: I am uninterested in discussions on whether the church is right or wrong. The commenter asked for an explanation of why the Church believes as it does, which I have given.

[1] http://www.vatican.va/roman_curia/congregations/cfaith/docum...