As CatholicCulture.com website says:
Here's what the Ethical and Religious Directives for Catholic Health Care Institutions (ERDs) from the United States Conference of Catholic Bishops say about it:
"A female who has been raped should be able to defend herself against a potential conception from the sexual assault. If, after appropriate testing, there is no evidence that conception has occurred already, she may be treated with medications that would prevent ovulation, sperm capacitation, or fertilization. It is not permissible, however, to initiate or to recommend treatments that have as their purpose or direct effect the removal, destruction, or interference with the implantation of a fertilized ovum."
A hospital in the Diocese of Peoria, Illinois, developed a protocol for Catholic hospitals that face this situation. Called (appropriately enough) the Peoria Protocol, it was put together by the staff at OSF St. Francis Medical Center and theologians in the diocese working under then-Bishop John J. Myers.
According to this theory, then, since the woman hasn't ovulated yet, the man's sperm can be considered an attacker, like bacteria. Since you can take prophylactic antibiotics, you can take anti-spermicide or anti-ovulatory agents. According to this theory, it isn't considered contraception since the sperm is in "attack" mode - I guess they have little green berets on or something.
The whole argument is absurd, and the monsignor who was instrumental in developing it (I knew him and worked with him on the chancery staff a few years after the protocol was developed), apparently neglected to consider the theology involved.
The only way a human life comes into existence is through God creating and infusing a human soul. If He doesn't do that, it doesn't matter how many sperm make it to the egg - no fertilization, no new life, will exist. Human life is the combination of soul and body. No soul, no human life, no embryonic growth, no way to implant, yada, yada, yada.
Now consider what would happen if no spermicide or anti-ovulatory agent is used and God never got around to creating and infusing the human soul. What would be the result? There would be no result. The sperm would do no harm to the woman. None. They would just curl up, die and disappear. They might make it to the egg, but without a human soul, what difference would that make? None. The sperm couldn't fertilize the egg in such a way that a new human life would appear.
So, you can certainly say the rapist or whoever is attacking the woman. You can certainly try to prevent that attack. But you can NEVER say the sperm is attacking anything, because the sperm is not capable of causing any damage. The worst it does, in conjunction with God, is cause a human life to begin.
And if we're going to call the creation of a new human soul and a new human life an attack, then God is a serial rapist.
So, the Peoria protocol is complete crap, as I said. Human life is always a gift, unless you get in front of this particular monsignor and the bishops foolish enough to follow him, in which case human life is sometimes the combination of an embryo and God gang-raping a woman.
But this is what passes for high-falutin' theological scholarship nowadays, don'tcha'know.