What Scripture Says

26 For this reason God gave them over to dishonorable passions. For their women exchanged the natural sexual relations for unnatural ones, 27 and likewise the men also abandoned natural relations with women and were inflamed in their passions for one another. Men committed shameless acts with men and received in themselves the due penalty for their error.Romans 1:26-27

There's really not much to debate here. The Bible explicitly condemns homosexuality as sin. In the Romans passage above, it's even indicated that homosexuality is a form of punishment for itself. Note that, contrary to common arguments, homosexuality is addressed in both the Old and New Testament. Here are several other passages that declare homosexuality to be sinful:

You must not have sexual intercourse with a male as one has sexual intercourse with a woman; it is a detestable act.Leviticus 18:22

If a man has sexual intercourse with a male as one has sexual intercourse with a woman, the two of them have committed an abomination. They must be put to death; their blood guilt is on themselves.Leviticus 20:13

Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived! The sexually immoral, idolaters, adulterers, passive homosexual partners, practicing homosexuals, 10 thieves, the greedy, drunkards, the verbally abusive, and swindlers will not inherit the kingdom of God. 11 Some of you once lived this way. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God.1 Corinthians 6:9-11

Objections

The Bible is speaking culturally!

No it isn't. Many claim that if Christians listen to Leviticus, then they must listen to all of it, and so stone adulterers, etc. While there are many problems with such an argument, we need only note that the condemnation of homosexuality is found in the New Testament, as well, so the case against homosexuality could be made without ever mentioning Leviticus. Also, note what the Bible says about marriage and sex, and how they, too, condemn homosexuality:

Sex only in marriage

Second, we should note the Bible's limits on sex:

Marriage must be honored among all and the marriage bed kept undefiled, for God will judge sexually immoral people and adulterers.Hebrews 13:4

Marriage only between a man and a woman

Sex outside of marriage is prohibited. If two homosexuals are "married," then, does this permit their homosexuality? No; the Bible teaches that marriage is between a man and a woman:

The Lord God said, "It is not good for the man to be alone. I will make a companion for him who corresponds to him."Genesis 2:18

Here we see that God wanted a suitable companion for Adam. In the following passage, animals were named, but they were not considered suitable. This same part of Genesis that condemns homosexuality also speaks against beastiality.

Then the man said,

"This one at last is bone of my bones
and flesh of my flesh;
this one will be called 'woman,'
for she was taken out of man."Genesis 2:23

So in order for God to provide Adam with a suitable mate, he created not another man, but a woman. God did not find it suitable to provide Adam with a another man.

That is why a man leaves his father and mother and unites with his wife, and they become a new family.Genesis 2:24

Again, the man is united with his wife, and this is traced back to ("This is why") God making Adam a woman to serve as his mate. Ephesians 5:22-33 explains the relationship between a man and a woman in terms of the relationship between Christ and the Church, even quoting Genesis 2:24. Clearly, God holds the marriage relationship to be very important. If the Bible speaks of marriage as between a man and a woman, and sex as being ok only as part of a marriage, then homosexuality is not condoned by God.

God says not to judge!

Do not judge so that you will not be judged.Matthew 7:1

I won't go into detail here, but this verse must be understood in context. Matthew 7 teaches us not to judge hypocritically. In fact, verse 5 goes on to say, "First remove the beam from your own eye, and then you can see clearly to remove the speck from your brother's eye." (Emphasis mine)

Also, we are to hate the sin, but love the sinner.

Interestingly, Lot was accused of being judgmental, as well, yet God was with him! Two angels, appearing as men, went to Sodom to warn Lot before God destroyed the city. The immoral men of the city demanded that Lot turn the angels over to them so that they could have sex with them. When Lot refused, calling such an action wicked, the men responded, "He dares to judge us!" (Genesis 19:9). The Christian should not be afraid to preach against sin. Christ himself spoke very much about sin, telling people to repent and sin no longer, being clear that hell awaited the unrighteous. Christians, as well, are commanded to rebuke sin:

Preach the message, be ready whether it is convenient or not, reprove, rebuke, exhort with complete patience and instruction.2 Timothy 4:2

The cry of the world that Christians hate homosexuals is a reflection of the world's own hate! While Christians are called to love adulterers, thieves, murderers, liars, and even evil dictators, the world is quick to hate such people. To the world, then, there is no way to hate what someone one does, yet still love the person doing it. To them, saying that we shouldn't look down on homosexuals and treat them poorly makes one "pro-gay," while speaking against homosexuality makes one a bigot. The Christian, however, does both, and this is beyond the world's comprehension.

But gays are born gay!

Studies have shown a strong pattern of homosexual behavior that seems family related, yes. However, the same sorts of studies have shown similar familial patterns of murder, rape, abuse, crime, etc. Do we excuse this behavior? Of course not! Nor should we excuse homosexuality.

The fact is, all men are sinful by nature. This sin expresses itself in all sorts of ways--lying, stealing, selfishness, gluttony, etc. In the case of a homosexual, it expresses itself in homosexuality. In this sense (speaking of a the sinful urge), Christians should not suggest that homosexuality is a choice. The urge to commit homosexual acts is part of the homosexual's sinful nature (all of us have a sinful nature, though it is expressed in different ways). Of course, homosexuality is a choice in that acting on these urges is akin to acting on the urge to punch someone in the face.

Furthermore, other sins could be justified this same way, if we were to allow the "I was born this way" argument. Does a married man choose to have sexual urges towards another woman? Does a selfish child choose to be selfish? In all of these cases, the effort and choice has to be made to go against the sinful behavior. This is, of course, perfectly in line with the teaching of the Bible, and contradicts the teaching of the world that men are essentially good by nature. While a homosexual may not choose to have homosexual urges, he certainly is responsible for acting on them, just as a married man is responsible for his cheating on his wife, a child responsible for being selfish, a glutton responsible for gorging himself, and a thief responsible for stealing.