These arguments may have some validity, but there is a strong biblical argument against homosexual activity. It is not from the texts cited above (although they are part of the evidence) but from the scriptural portrayal of heterosexual fidelity as God's ideal for humans, from the earliest parts of Genesis to the portrayal of the church as the bride of Christ in Revelation. (See Genesis 2:18-24, Exodus 20:14, Proverbs 5, The Song of Solomon, Ezekiel 16, Hosea 1-3, Matthew 19:4-6, John 2:1-11, Revelation 19:6-8, and elsewhere.)
1. Is homosexuality wrong? Yes and no. Based on scripture, homosexual activity is wrong. Homosexual tendencies are not wrong, any more than heterosexual ones, unless those tendencies are due to wrong choices. If I am attracted to someone other than my wife, because I have heterosexual tendencies, and act on it, that's wrong. Having the tendencies isn't wrong. (Acting on it doesn't mean just adultery or fornication--deliberately exposing myself to pornography, or lusting after movie stars, etc., are ways of acting on heterosexual tendency, and acting sinfully.) It isn't wrong to have heterosexual tendencies, unless those tendencies are due to wrong choices, so why should it be wrong to have homosexual tendencies?
2. Is homosexual tendency built in? In some cases. Recent evidence indicates that pre-natal hormonal exposure is important. There is probably some genetic influence. But some people choose homosexuality over heterosexuality. (James Dobson believes that homosexuality is due to how a child was raised, which is probably part of the story. If it were all of the story, the cause would be neither a choice by the person or some built-in factor.)
3. Isn't having homosexual tendencies, but not being able to act on them without sinning, unfair? God is not ever unfair. He may demand more of some than others, in certain aspects of their lives. All of us are born with tendencies that we must control in order to live Christian lives. It isn't just homosexuals that are called to life-long celibacy -- some heterosexuals are. All heterosexuals are, until they are married.
4. Can homosexuals form long-lasting same-sex relationships? Apparently. However, especially with males, this often doesn't happen. (It often doesn't happen with heterosexual marriage relationships, either, but that's another sad story.) Are such relationships identical to stable Christian heterosexual marriages, in God's sight? No. They aren't God's plan. You could have a long-lasting sexual relationship with a prostitute, or even a dog, but the fact that it's long-lasting doesn't make it right.
5. Can homosexuals become happy heterosexuals? At least some of the time. (See 1 Corinthians 6:9-11.) Probably not all of the time.
6. What should be the Christian attitude toward homosexuals? Practicing homosexuals, like practicing gossips, gluttons or thieves, are sinners. We should love them, but not love their sin. Overt sinners should not be leaders in churches, and certainly not pastors.
7. Is homosexuality the worst threat to marriage in North America? No. If there were no homosexuals at all, marriage would still be under continuing and violent attack from its real worst enemy, namely that a woman and a man don't make Jesus Christ Lord of their relationship.
8. Is homosexual activity the worst sin? No. See what Jesus said in comparing Sodom to the people of his day, in Matthew 11:20-24, and Luke 10:1-12. Romans 1 indicates that homosexual behavior is a symptom of a worse sin, idolatry or unbelief.
Homosexual activity is not even the worst sexual sin -- it's not part of the 10 commandments. There's a list of curses for sinful activity in Deuteronomy 27:15-26. Four such were curses for sexual misconduct, and they didn't include homosexual activity. (That does not, of course, make homosexual activity acceptable for Christians.)
The above is a re-do of a previous post, done, I believe, in 2009. On January 24, 2013, I added the reference to Deuteronomy 23, and made an editorial change to the statement in bold at the beginning of this post. On March 6, 2013, I edited the post, cleaning up some errors.
There are some other topics that I wish to mention briefly.
Intersexuality, or ambiguous anatomical sexual anatomy
(See Wikipedia article.) There is controversy over classification issues, but it seems that from 0.05% to as many as 1.7% of babies born have genital anatomy that is not normal. What does that have to do with homosexuality? This -- not everyone is born "normal." People do not choose to be born with ambiguous external genitals, nor do they become this way because of the way that their parents raised them, which relates, at least somewhat, to the questions of how people become homosexual, and God's fairness to them. So far as I know, Christian thinkers have not considered this phenomenon at all, let alone in depth.
Should Christians oppose civil unions? A civil union would give homosexual couples some legal rights, such as joint ownership of property, and hospital visitation rights, but would not have the same type of recognition as a marriage. This is a difficult question, and I haven't resolved it in my own mind. If I am offered the chance to vote on the issue, I will try to make up my mind, God helping me. On the one hand, it seems unfair to deny a person, who has been a caregiver, hospital visitation rights for the person he or she has cared for, regardless of their sexual preferences. On the other hand, this might be a step toward full recognition of marriage between homosexuals, on the same basis as between heterosexuals, and, as indicated above, I do not believe that a homosexual couple can have God-approved marriage.
A church-approved marriage does not need to be the same as a state-approved marriage, although, often, it is. Couples, even totally unchurched couples, often seem to believe that they have a right to a church wedding, and churches often comply, sometimes even without any pre-marital counseling. Probably many churches, including my own, have, occasionally, had marriage ceremonies between a man and a woman performed within them that united couples that God really didn't want together.
Should Homosexuals be allowed to join churches?
I would draw upon the example of the early church, except that I am not clear as to whether the early church had membership in the same way that many churches do today. Let me stipulate that a church member is someone that the congregation, or its leaders, believes is a converted Christian, who is in agreement with basic Bible doctrine, and Christian practice, as understood by that church.
I don't see any reason to deny membership to a person with homosexual tendencies, as indicated above. Based on 1 Corinthians 6:9-11, persons engaged in homosexual activity should not be taken into membership, any more than greedy people, or swindlers, should be. They are mentioned in the same list as homosexuals. The good news is that the same passage states that some of the current members of the Corinthian church used to do some of these things, but had been redeemed from such activities, presumably including homosexuality.
Thanks for reading.
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Added Jan 26, 2013: Here's a later post, on adultery and fornication.
Added July 29, 2014: Here's a later post, on the idea that Jesus referred to homosexuality without condemning it.
Added January 8, 2015: Here's my post on what's wrong with the political left/right in the US
Added January 18, 2015: One of the possible dangers, for the near future, for Christians, is that merely saying that the Bible says that homosexual activity is sinful may be called illegal hate speech, no matter what the motivation or tone of the statement. In other words, even answering a question: "Does the Bible say that homosexual activity is sinful?" with a "Yes, I believe that it does" may lead to legal action.
Added May 27, 2016: Benjamin L. Corey writes that some Christians are changing their attitude toward homosexuals, and indicates why this is so. (He thinks it's a good thing, by the way.)
On June 8, 2016, I added scriptural references to the first paragraph. I recommend this post on scripture and homosexuality by Ken Schenck.