The idea is not blasphemous to me, merely bizarre. We have an account of a man who considered himself free to journey around the country conducting what was for all practical purposes a religious revival for three years. In the social condition of first century Galilee, who’s supporting his wife and kids while he does this? And why do we have absolutely no evidence except the ultracryptic garbage of Holy Blood, Holy Grail and The DaVinci Code to prove it? People who have a hard time accepting the possibility that he existed at all are jumping to saddle him with a nuclear family – producing the truly peculiar picture of a woman who bears children to a myth. The evidence is nearly as good as the scenario of a few years back that having him traveling to Tibet to study Lamaistic Buddhism (which is I believe a trifle anachronistic).
For what it’s worth, the Catholic (and I think Orthodox) doctrine of Mary’s Perpetual Virginity (which is not held by other groups) probably owes more than a little to a sense of sinfulness in sexuality by those who hold it. But it is not theologically grounded in that idea, but in the original sin/Immaculate Conception/sacramental nature of marriage complexus of ideas. Like this:
All men and women, according to Catholic doctrine, are conceived and born in original sin, the common sinful heritage of humanity. This is distinct from actual sin but serves the purpose of separating the “natural” man (as opposed to one regenerated in baptism) from God. A woman with the heritage of original sin, however holy she might be in her effort to live a good life, would not be a fit receptacle for the conception and birth of God the Son, the Savior Jesus Christ. By a special act of grace, therefore, Mary was conceived (as a result of normal sexual intercourse between her parents) free from the taint of original sin. This is the much-misunderstood Immaculate Conception.
But in marriage, specifically in the conjugal act, husband and wife become one flesh. Joseph, while specifically called “a good man” in Scripture, was tainted by the common curse of humanity, original sin. For him and Mary to have had sexual relations, therefore, while in no way sinful but the proper behavior for a married couple, would mean that the sinless Mother of God and her sin-bearing husband would become one flesh. To preserve the miracle of her sinlessness, they voluntarily abstained from marital sex (otherwise their proper right and duty to each other) and kept her perpetually virgin.
As an Anglican, I do not hold to this belief. But in all fairness to Catholics, I think it’s only fair to explain it. (Note that while the Orthodox believe Mary to have been perpetually virgin, they do not buy into the Immaculate Conception/original sin system outlined here, which is specifically Catholic.)