The Catholic Church thinks that sex is very holy when it is sacramental. That is why there is a Sacrament of Matriomony.
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Perhaps you are not familiar with Pope John Paul II's book on the Theology of the Body. It speaks about the sacredness of the sexual act, the sacredness of the marital union, etc.
For your information:
Christopher West's Theology of the Body Webiste
Books, tapes, CDs, DVDs, etc.
From book by the same name by Pope John Paul II.
So WHY is it NECESSARY for Mary to remain a perpetual virgin ? If marriage and sex are holy, why can Mary only be loved and honored if she never had sex?