It is a sin, according to the Bible, for two people of the same sex to be intimately involved in a sexual relationship with one another (a relationship which one would only expect to see between a husband and wife). In fact, there are many verses in the Bible that speak of this. (Leviticus 18:22; 20:13; Romans 1:26-27; 1 Corinthians 6:9-10)

It would also seem to be that homosexuality was a major factor in the destruction of the cities of Sodom and Gomorrah, and the surrounding areas, by God. (Genesis 19:4-5) As to one living a gay lifestyle, and forgiveness from God, it's important to understand that the first step to forgiveness is repentance. (Luke 17:3) Repentance is a change of direction, or a turning away from known sin in our life.

In the early Christian Church (in Corinth) there were those who, at one time in their life, had lived a gay lifestyle. (1 Corinthians 6:9)  Even so, when they turned to God in repentance through faith in Christ Jesus, God forgave them their sin, and adopted them into His family (1 Corinthians 6:11; John 1:12); the same is true today.

There is no sin so large that God is not willing, and able, to forgive us of it when we truly turn to Him in repentance, desiring His forgiveness.