Traditionally, the Old Law is divided into three parts: the ceremonial law, the disciplinary law, and the moral law.
- The ceremonial law encompassed all of the ceremonies and sacrifices: what kinds of animals to sacrifice, what parts to burn, what parts belonged to the priest, how often to sacrifice, etc. The most important part of the ceremonial law was the Day of Atonement (Yom Kippur), described as the Sabbath of Sabbaths. On this day, the high priest would enter the holiest part of the temple and offer animals in atonement for the sins of Israel (see Leviticus 16)
The ceremonial law was superseded by Jesus, who is High Priest, Sacrifice, and Altar of the New Covenant. Now, instead of sacrificing animals, which could not take away our sins, Catholic priests, acting in persona Christi, offer the same sacrifice of Calvary to the Father (see the letter to the Hebrews, especially chapter 9).
- The disciplinary law encompassed the dietary restrictions, what kind of clothes to wear, and other related things. These things (including the oft-quoted examples of shellfish, mixed fabrics, and shaving) were specific to the Jews, and never applied to the rest of the world.
One part of the disciplinary law that was more important was the penalties for breaking other laws; some commonly quoted examples are the death penalty for disobedient children and a simple fine for rapists. Although the moral laws they enforced (obey your parents, don’t rape) are still valid, these particular penalties were intended only for the Jews.
The disciplinary laws were abrogated by Jesus (see Mark 7:1-23, especially verse 19), and the early church declared that they didn’t apply to Gentiles (Acts 15).
- Unlike the ceremonial and disciplinary laws, the moral law is eternal, and applies for all time to all peoples. It is codified above all in the Ten Commandments, and in such related rules as the Shema or the commandment not to oppress widows and orphans, who were usually the poorest and most oppressed classes. Rather than abrogating these laws, Jesus took these commandments even further when he declared that anyone who was angry with someone else was guilty of transgressing the whole Law.
How do we know that the idea of marriage as one man and one woman is part of the moral law, and not the disciplinary law?
We know what marriage is supposed to be not just because of scripture, but also because of reason. Shortly after talking about the unnaturalness of same-sex sexual relations, Paul tells us that the Law is written in our hearts. The Church calls this the Natural Law, and insists that we can know this Law through reason. This Law tells us that marriage is for the love and mutual betterment of the spouses, but also for the procreation and education of children. Since a same-sex union could never result in a child, it does not qualify as a marriage; whereas an opposite-sex union has that possibility, even if one or both partners is infertile, and so is a true marriage.
So, no, we’re not hypocrites, and we don’t pick and choose which parts of the Bible to follow. Rather, we follow our own logical, millenia-old theology