A semen allergy is not impotence. What you are stating is completely offensive. Going by Catholic principals, if there are no other options, this would seem to follow under the principal of double effect. A piece of latex is not objectively immoral. Intentionally objectively thwarting the reproductive function of the marital act is. IOW, contraceptives are immoral. “the agent intends the good effect and not the bad either as a means to the good or as an end itself;”
Humanae Vitae covers the principal of double effect:
On the other hand, the Church does not consider at all illicit the use of those therapeutic means necessary to cure bodily diseases, even if a foreseeable impediment to procreation should result there from—provided such impediment is not directly intended for any motive whatsoever.
"In 1987, the U. S. Conference of Catholic Bishops issued a document suggesting that education on the use of condoms could be an acceptable part of an anti-AIDS program. Cardinal Ratzinger responded:
" In the first place, and on a more general level, one must keep in mind the problem posed by the worldwide reaction which accompanies certain documents issued by various episcopal conferences. "
In other words, its like women wearing head covers during Mass. The Church never got rid of it. News outlets misconstrued what we being conveyed by the Church and told Catholics that women would no longer be required to wear headcoverings in Mass. The Church today has simply decided the issue is not important enough to address. The important thing to recognize is that when the Church clarifies a moral teaching publically, the media take it, miscontrue it and the faithful get the wrong message. The contraceptive issue and the issue of HIV is an issue that needs clarity beyond some soundbite or short paragraph a newspaper would offer. The Church needs to be careful because the teaching against contraceptives needs to be emphasized.
He then quotes the Vatican Newspaper: "To seek a solution to the problem of infection by promoting the use of prophylactics would be to embark on a way not only insufficiently reliable from the technical point of view, but also and above all, unacceptable from the moral aspect. Such a proposal for ‘safe’ or at least ‘safer’ sex – as they say – ignores the real cause of the problem, namely, the permissiveness which, in the area of sex as in that related to other abuses, corrodes the moral fiber of the people. " Ratzinger adds: "In a society which seems increasingly to downgrade the value of chastity, conjugal fidelity and temperance, and to be preoccupied sometimes almost exclusively with physical health and temporal well-being, the church’s responsibility is to give that kind of witness which is proper to her, namely an unequivocal witness of effective and unreserved solidarity with those who are suffering and, at the same time, a witness of defense of the dignity of human sexuality which can only be realized within the context of moral law. It is likewise crucial to note, as the board statement does, that the only medically safe means of preventing AIDS are those very types of behavior which conform to God’s law and to the truth about man which the church has always taught and today is still called courageously to teach. "
In other words, the problem with condoms and aids prevention is that condoms do not sufficiently protect people from the HIV virus to fit under the principal of double effect. Its safer sex, not safe sex. The promotion of condoms encourages that unchaste behavior that fuels the continual spread of the virus. Especially in Africa, one of the primary problems with the spread of HIV is the post-partem sex taboo. There are superstitions that if husband and wife have sex within (I’ve heard it can be as long as 2 years, but it depends on who you talk to) a certain time frame, something bad will happen to the baby. As such, marital infidelity is a huge problem. The culture in fact relies a lot upon marital infidelity to follow this post partem sex taboo. The women can actually be faced with “Its unrealistic for my husband to be expected to abstain. He will have sex with other women and I will continue to refuse him sexually if he asks during the tabooed period because I don’t want it to harm our baby.” Of course the promiscuity and infidelty is what is actually harming Africans. A condom isn’t going to solve the larger issues going on.