ANSWER: Webster's Dictionary defines sodomy as having sex (either orally, through the anus, etc.) with a person of the same gender or with any kind of animal. This description roughly coincides with how the Bible defines the term when it uses it.
As early as the time of Abraham and Lot (roughly 1900 B.C.) Sodomy was considered unacceptable and 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 usually alludes to this behavior, it without a doubt condemns the practice (Genesis 13:13, 19:5, 19:7, Leviticus 18:22, 20:13 and so on). It was a particular manifestation of wickedness through which the city of Sodom became notorious (and from which the term originated).
This sin is an offense against nature which many times was 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 God, contain his instructions for all mankind and not just for Levitical priests or Israelites. It says the following about sodomy between humans and includes what we would call bestiality.
13 If a man has sexual relations with another man, they have done a disgusting thing, and both shall be put to death. They are responsible for their own death. 15 If a man has sexual relations with an animal, he and the animal shall be put to death. 16 If a woman tries to have sexual relations with an animal, she and the animal shall be put to death. (Leviticus 20:13, 15 - 16)
22 No man is to have sexual relations with another man; God hates that. 23 No man or woman is to have sexual relations with an animal . . . (Leviticus 18:22-23)
The New Testament tells us directly what happens to those who reject the knowledge of God and His command that this type of behavior is a sin.
24. For this cause, God also abandoned them to uncleanness through the lusts of their hearts, to disgrace their own bodies between themselves . . . 28. 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, 28, HBFV)
In conclusion, sex through the anus within marriage (man and woman) is not directly prohibited in the Bible. The practice of sodomy, however, God clearly condemns.