The Church not only approves of adoption, but in light of Pope St. John Paul II’s 1995 encyclical Evangelium Vitae (“The Gospel of Life”), the Church encourages adoption, especially if the married couple has a stable and strong relationship. Even if a couple has already been blessed with natural children, some have the ability to adopt children who might otherwise be deprived of a loving home. Adoption is not the optimal choice for all couples, and the decision should be made only after prayerful and prudent consideration, but those who can adopt children out of a spirit of generosity do a good thing.
Infertility could be a compelling reason to adopt, and so the instruction Dignitatis Personae (2008) states: “In order to come to the aid of the many infertile couples who want to have children, adoption should be encouraged, promoted and facilitated by appropriate legislation so that the many children who lack parents may receive a home that will contribute to their human development.”
The Church prohibits in vitro fertilization for two reasons: First, it separates the unitive from the procreative aspect of the marital act; second, it leads to the destruction of embryos, which are, after all, tiny human beings. The Church stated the prohibition in 1987 with the instruction Donum Vitae, and it was reiterated in Dignitatis Personae.
As for in utero fertilization, you refer to fertilization techniques which help couples conceive with the aid of some technological assistance, such as artificial insemination after a natural marital act. Some of these techniques have not been prohibited by the Church, although pious commentators wonder if the dignity of the persons is truly respected when the modesty and intimacy of husband and wife are unveiled by a well-meaning but intrusive third party dressed in a white lab coat.
The marital act has two objects: the procreation of children and the intimate expression of exclusive spousal love. Introducing a third party into the operation may strengthen the first object (procreation), but at the expense of shattering the second object (the intimate expression of exclusive spousal love). I am not convinced it is worth it.
Here it may be helpful to stress what Dignitatis Personae points out: 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; 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.” Techniques which assist procreation “are not to be rejected on the grounds that they are artificial. As such, they bear witness to the possibilities of the art of medicine. But they must be given a moral evaluation in reference to the dignity of the human person, who is called to realize his vocation from God to the gift of love and the gift of life.”
Marriage may still be advisable even if one of the would-be spouses were infertile, unless the other spouse specifically wanted to have children with that spouse. Not infrequently, widows and widowers marry even though they are beyond the age of fertility. And marriage in that situation is a very good thing.