Question:
Answer:
They are surely permitted to marry and to have the use of marriage, but there would be reasons, perhaps, to counsel against marrying. At a minimum both the man and the woman should be fully informed of the health of the other. If they marry, they are permitted to have sexual relations as any couple, but they are not allowed to use contraception. It is possible that they might use some means of minimizing the risk of infection, but this must not hinder the natural possibility of conception.
This answer may seem irresponsible to some people, but we are speaking here of a fundamental human good, and so the presumption is in favor of permitting marriage.
As medical science becomes more exact, there will be more cases where the couple can know of some genetic predisposition toward a disease or defect. This does not in itself, however, deprive a couple of the right to marry and to have intercourse. It is not the position of Christ or of the Church that it is better not to exist at all rather than be born with some illness or defect.
This being said, one must take into account the burden of bearing with some serious condition. This may strongly indicate that the couple should refrain from marriage or intercourse, but this decision must be made taking everything into account before God. There is no automatic answer to these difficult situations.