Technically, mixed marriages are those between Catholics and non-Catholics, when the latter have been baptized in some Christian sect. The term is also frequently employed to designate unions between Catholics and infidels. From the very beginning of its existence the Church of Christ has been opposed to such unions. As Christ raised wedlock to the dignity of a Sacrament, a marriage between a Catholic and a non-Catholic was rightly looked upon as degrading the holy character of matrimony, involving as it did a communion in sacred things with those outside the fold. The Apostle St. Paul insists strongly on Christian marriage being a symbol of the union between Christ and His Church, and hence sacred. The very intimacy of the union necessarily established between those joined in wedlock requires a concordance above all in their religious sentiments. Holding this doctrine, it was but natural and logical for the Church to do all in her power to hinder her children from contracting marriage with those outside her pale, who did not recognize the sacramental character of the union on which they were entering (see Marriage). Hence arose the impediments to a marriage with a heretic (mixta religio) and with an infidel (DISPARITAS CULTUS). As regards marriage with an infidel, the early Church did not consider such unions invalid, especially when a person had been converted to the faith after such marriage.

http://www.newadvent.org/cathen/09698a.htm

can't argue with the Vatican :-)