Answer: Homosexuality (commonly referred to as being gay), as defined in the Bible, is sexual desire or intercourse directed toward someone of the same gender (Leviticus 18:22).
It should first be noted that the King James translation does not contain the words "homosexuality" or "gay" in its text. The word "gay" comes from the Middle English word "gai" which came into existence around 1275 to 1325 A.D. (dictionary.com). It originally meant something that was fast, sudden, merry or showy. The use of the word gay as a synonym began sometime around the late 1930s A.D.
Why is it unacceptable?
Why does God detest homosexuality? Paul, in the book of 1Corinthians (see 1Corinthians 15:42 - 49), reveals that the Eternal is reproducing Himself by creating spiritual sons and daughters. Our ultimate destiny, if we are willing and repent of our sins, is to be a part of the divine family. The purpose of marriage and sex, between a man and a woman, is to produce children that can also ultimately enter the Kingdom of God.
A lifestyle that centers on being gay works against God's great plan since procreation cannot occur between two people of the same sex. It is confusion because God ordained reproduction through kind after its kind (Genesis 1).
The Bible defines and clearly states that homosexuality is forbidden and an abomination in the eyes of God (Leviticus 18:22, 20:13, 1Timothy 1:8 - 10). His judgment against those caught practicing this behavior was the death penalty (Genesis 19, Leviticus 20:13). Those who continuously indulge in the fulfillment of such lusts and do not repent will not be in the Kingdom of God (1Corinthians 6:9 - 11). Those who repent of their sinful ways, however, will be forgiven by God through Christ.