newadvent.org/cathen/09707a.htm
As we have several times emphasized, not even marriage is a true sacrament, but only marriages and validly between Christians. One becomes and remains a Christian in the sense recognized here through valid baptism. Hence only one who has been validly baptized can contract a marriage which is a sacrament;
but every one can contract it who has been validly baptized, whether he has remained true to the Christian faith, or become a heretic, or even an infidel. Such has always been the teaching and practice of the Church. Through baptism one “becomes a member of Christ and is incorporated in the body of the Church”, as declared in the Florentine Decree for the Armenians; so far as law is concerned, he remains irrevocably subject to the Church, and is therefore, in legal questions, always to be considered a Christian. Hence it is a general principle that all baptized persons are subject to universal ecclesiastical laws, especially marriage laws unless the Church makes an exception for individual cases or classes. **Hence not only the marriage between Catholics, but also that contracted by members of the different sects which have retained baptism baptize, is undoubtedly a sacrament. It matters not whether the non-Catholic considers marriage a sacrament or not, or whether he intends to effect a sacrament or not. Provided only he intends to contract a true marriage, and expresses the requisite consent, this intention and this expression are sufficient to constitute a sacrament. **But if he is absolutely determined not to effect a sacrament, then, of course, the production of a sacrament would be excluded, but the marriage contract also would be null and void. By Divine ordinance it is essential to Christian marriage that it should be a sacrament; it is not in the power of the contracting parties to eliminate anything from its nature, and a person who has the intention of doing this invalidates the whole ceremony. It is certain, therefore, that marriage contracted between baptized persons is a sacrament, even the so-called mixed marriage between a Catholic and a non-Catholic, provided the non-Catholic has been validly baptized.