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.¹

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.²

As to one living a gay lifestyle, and forgiveness from God, it's important to understand that the first step to forgiveness is repentance.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.⁴  Even so, when they turned to God in repentance through faith in Christ Jesus as Lord and Savior, God forgave them their sin, and adopted them into His family; the same is true today.⁵

There is no sin that is so large that God is not willing and able to forgive us our sin; when we truly turn to Him in repentance, desiring His forgiveness.⁶

Bible References:

¹ (Leviticus 18:22, 20:13; Romans 1:26-27; 1 Corinthians 6:9-10)

² (Genesis 19:4-5)

³ (Luke 17:3)

⁴ (1 Corinthians 6:9)

⁵ (1 Corinthians 6:11; John 1:12)

⁶ (1 John 1:9; Ephesians 1:7)