Excommunication is the most serious ecclesiastical penalty levied against a member of the Roman Catholic Church. It is a seldom used punishment to discipline unrelenting defiance or other serious violations of church rules, especially by those who are accused of "spreading division and confusion among the faithful" -- meaning, in practice, that the option of excommunication is more likely to be enforced when the disobedient Catholic is a visible and presumably influential public figure (such as a politician), but only rarely in the cases of non-public figures. Excommunication is never a merely "vindictive penalty" (designed solely to punish), but is always a "medicinal penalty" intended to pressure the person into changing their behavior or statements, repent and return to full communion.
Excommunicated persons are barred from participating in the liturgy in a ministerial capacity (for instance, as a reader if a lay person, or as a deacon or priest if a clergyman) and from receiving the eucharist or the other sacraments, but is normally not barred from attending these (for instance, an excommunicated person may not receive Communion, but would not be barred from attending Mass). Certain other rights and privileges are revoked, such as holding ecclesiastical office.
Excommunication can be incurred either ferendae sententiae (imposed or declared as the sentence of an ecclesiastical court) or latae sententiae (automatic, incurred at the moment the offensive act takes place).
In the Roman Catholic Church excommunication is usually terminated by a statement of repentance, profession of the Creed (if the offense involved heresy), or a renewal of obedience (if that was a relevant part of the offending act) by the person who has been excommunicated; the lifting of the excommunication itself, by a priest or bishop empowered to do this; and then the reception of the sacrament of penance. In many cases, this whole process takes place within the privacy of the confessional and during the same act of confession.
Offenses that incur excommunication must be absolved by a priest or bishop empowered to lift the penalty. This is usually the local ordinary (bishop or vicar general) or priests whom the local ordinary designates (in many dioceses, most priests are empowered to lift most excommunications otherwise reserved to the bishop, notably that involved with abortion).
The excommunicant is still considered Christian and a Catholic as the character imparted by baptism is held to be indelible. Excommunicated Catholics are still under obligation to attend Mass, even though they are barred from receiving the Eucharist or even taking active part in the liturgy (reading, bringing the offerings, etc.)
Indeed, the excommunicant is encouraged to retain the relationship with the Church, as the goal is to encourage them to repent and return to active participation in its life.
Catholics cannot be excommunicated unless for some personal, grievously offensive act. Here, therefore, it is necessary to state with precision the conditions under which this penalty is incurred. These are a few offenses for which Roman Catholics are automatically excommunicated:
1) Apostasy (canon 1364), is a term generally employed to describe the formal renunciation of one's religion, especially if the motive is deemed unworthy
2) Heresy (canon 1364). Heresy is defined by Thomas Aquinas as "a species of infidelity in men who, having professed the faith of Christ, corrupt its dogmas." The Catholic Church teaches that its doctrines are the authoritative understandings of the faith taught by Christ and that the Holy Spirit protects the Church from falling into error when teaching these doctrines. To deny one or more of those doctrines, therefore, is to deny the faith of Christ. Heresy is both the non-orthodox belief itself, and the act of holding to that belief.
3) Schism (canon 1364). An individual who withdraws from communion with the authorities of a Church, but who is neither expelled from it nor formally breaks with it, remains a member, though a disobedient one.
4) Desecration of the Eucharist (canon 1367),
5) Physical violence against the Pope (canon 1370)
6) Attempted sacramental absolution of a partner in a sin against the sixth commandment of the Decalogue ("Thou shalt not commit adultery.") (canon 1378 §1),
7) Ordination of a bishop without papal mandate (canon 1382),
8) Direct violation of the sacramental seal of confession by a confessor (canon 1388),
9) Procurement of a completed abortion (canon 1398),
or
Being a conspiring or necessary accomplice in any of the above (canon 1329). Any Catholic who obstinately denies that abortion is always gravely immoral, commits the sin of heresy and incurs an automatic sentence of excommunication.
Unfortunately, some Catholics obstinately deny that abortion is always immoral, and some Catholics claim that abortion can, at times, be a morally-acceptable choice, and some Catholics claim that a person can, in good conscience, choose abortion. Under the Code of Canon Law of the Roman Catholic Church, canons 751 and 1364, all such Catholics are automatically excommunicated for the sin of heresy.
Any Catholic who deliberately and knowingly obtains a procured abortion commits a mortal sin and is also automatically excommunicated, under canon 1398.
Under the laws of secular society, if one person commits a crime, then anyone who deliberately and knowingly provides essential or substantial means for that person to commit that crime is called an accessory to that crime and is also subject to the penalties of law. Similarly, any Catholic who deliberately and knowingly provides essential or substantial means for any woman to procure an abortion also commits a mortal sin and also incurs the same sentence of excommunication.
Any Catholic who substantially assists another in the deliberate sin of abortion is also guilty of serious sin and also incurs a latae sententiae excommunication.
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The above is what I've found so far about Ex-communication & the Catholic Church. It doens't say anything specifically about getting being ex-communicated due to getting married outside of the Catholic Faith. Maybe it falls under category#6, but that's only if one is not married, but live as if he/she does. Just to be sure I've sent an email to my parish priest asking about your specific question... I'll let you know if I hear from him.
