A Capital Sin
Sodomy, according to a 1913 Catholic Encyclopedia article, is considered a capital sin. Capital sins (according to Catholics) are those that, while they are pursued, lead to the committal of other sins. Other than lust, under which sodomy is considered a sub-category, the other capital sins are envy, gluttony, greed, pride, sloth and wrath.
Interestingly, in another 1913 Catholic article related to sex, they state that "most theologians" believe that even anal sex within marriage (which they label as sodomy) constitutes adultery and is therefore a sin.
The word sodomy does not appear in the King James Version of the Bible, as well as the NKJV, NASB, NIV and at least several other translations. According to dictionary.com, this English word did not exist before the 1300's A.D.
Webster's Dictionary defines sodomy as having sex (either orally, through the anus, etc.) with a person of the same gender (e.g. homosexuality and lesbianism) or with any kind of animal (bestiality). This explanation of the term is roughly supported in the Scriptures.
Biblical Examples
As early as Abraham and Lot (roughly 1900 B.C.), sodomy was considered a sin (Genesis 19:1 - 7). God also reiterated that this practice was against His will when He gave His laws to Israel. Although the Bible sometimes alludes to this behavior, it clearly condemns the practice (Genesis 13:13, 19:5, 19:7, Leviticus 18:22, 20:13 and so on).
The city of Sodom was so well known for its open and shameless indulgence in this wicked behavior that it lent its name to those throughout history who pursue it (sodomites).
This sin, an offense against nature, was many times connected with idolatrous practices. This is why the Biblical term "sodomite" (Strong's Concordance #H6945 as found in Deuteronomy 23:17, 1Kings 14:24, 15:12, 22:46, 2Kings 23:7, etc.), in the Hebrew, means a (male or female) devotee (prostitute) to licentious idolatry. Although allowed and encouraged in some non-Biblical or non-Christian religions, it was never tolerated in the worship of the true God.
The book of Leviticus, which was given directly by the Eternal, contains his instructions for all mankind and not just for Levitical priests or Israelites. It states the following about sodomy between humans and includes what we would call bestiality.
No man is to have sexual relations (sodomy) with another man; God hates that . . . If a man has sexual relations with another man, they have done a disgusting thing, and both shall be put to death . . .
If a man has sexual relations with an animal, he and the animal shall be put to death. If a woman tries to have sexual relations with an animal (a form of sodomy), she and the animal shall be put to death (Leviticus 18:22, 20:13, 15 - 16).
The New Testament clearly reveals directly to us what happens to those who reject the knowledge of God and His command that this type of behavior is a sin.
For this cause, God also abandoned them to uncleanness through the lusts of their hearts, to disgrace their own bodies between themselves . . .
And in the same manner also the men, having left the natural use of sex with the woman, were inflamed in their lustful passions toward one another - men with men shamelessly committing lewd acts, and receiving back within themselves a fitting penalty for their error.
And in exact proportion as they did not consent to have God in their knowledge, God abandoned them to a reprobate mind, to practice those things that are immoral . . . (Romans 1:24, 27 - 28).
Conclusion
The Biblical definition of Sodomy is any copulation (sex) with someone of the same gender whether or not society considers him or her "legally" wed. This act is clearly condemned in God's word. Sex through the anus between a man and woman, however, in marriage, is not clearly prohibited in Scripture (though it may not be wise to do so). This might be because the marriage bed of two people, whose relationship is sanctioned before God, is considered undefiled (see Hebrews 13:4).