[quote=Illini]Should (or may) a Mass be omitted and only the nuptials performed if the couple is mixed, that is, one is Catholic and the other is a baptized Protestant?
A couple is not required to have a Mass in conjunction with their nuptials, although it is certainly fitting for them to do so if both partners are Catholic. If they prefer, they may only have the marriage ritual and it may be presided by a deacon.
[quote=Illini]Does the answer change if the marriage is between a Catholic and Orthodox, or Catholic and non-baptized?
A Catholic and Orthodox may be married in a Catholic nuptial Mass if both parties desire it and if the usual dispensation from cult is obtained. However, an Orthodox Christian is usually urged by his own church not to receive Catholic Communion and not to marry outside his Orthodox church. An Orthodox Christian should speak to his priest about the requirements he must meet to marry a Catholic and remain in good standing in his Orthodox church. A Catholic could be married in an Orthodox ceremony provided he obtained a dispensation from form to be married in a non-Catholic ritual.
As for a marriage between a Catholic and an unbaptized person, the Church usually does not allow a nuptial Mass for such weddings. If they wish to marry in the Catholic ritual, they do so in the marriage ritual without the Mass. A deacon may preside at such a wedding, as well.