The Bible is clear that homosexual behaviour is a sin, just as fornication and adultery are.

1Co 6:9  Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites,

The Old Testament uses strong language, calling it an abomination.

Lev 18:22  You shall not lie with a male as with a woman. It is an abomination.

Some people feel very strongly that they are born with their sexual orientation and therefore it is unfair of God to make them homosexual and condemn homosexual behaviour.  There is much debate about whether people are born homosexual or develop into it.  Other than when God “gives” people up to such behaviour, the Bible doesn’t touch on the origins of homosexuality.

Rom 1:26  For this reason [for knowingly rejecting God] God gave them up to vile passions. For even their women exchanged the natural use for what is against nature.
Rom 1:27  Likewise also the men, leaving the natural use of the woman, burned in their lust for one another, men with men committing what is shameful, and receiving in themselves the penalty of their error which was due.

Regarding sex and sexuality, the focus of the Bible is not on its origins, but on its use.  Regardless of sexual orientation, temptations always exist.  A heterosexual can be tempted to commit fornication or adultery just as a homosexual can be tempted to commit sodomy.  The temptation is not sin; that’s just part of life.  Giving in to the temptation is sin.

The good news is that God offers forgiveness when we repent; and when we desire to obey God, He promises us help through the Holy Spirit.