Leviticus 18:22 (also 20:13)
Do not lie with a man as one lies with a woman; that is detestable.
1 Corinthians 6:9-10 (also Romans 1:27)
Neither the sexually immoral nor idolaters nor adulterers nor male prostitutes nor homosexual offenders nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God.
Does Christianity really teach that homosexuality is wrong?
The articles below cover most of the arguments and counter-arguments on this topic.
What if homosexuality is biologically determined?
Christians are often hateful and abusive to homosexuals - isn't this wrong?
Of course it's wrong! Unfortunately, Christians, like all people, don't always do the right thing or live up to their ideals. For evidence that abuse of homosexuals is against Biblical teaching, see Does Christianity endorse killing nonbelievers?
Other responses (offsite):