hat does the Bible say about how to lovingly show support for a person while not supporting the sin that they are living in?

This is an incredibly difficult decision to consider, and by no means will this be the end-all of the conversation. First of all, there is the reality of what the Bible says about homosexual, or same-sex, relationships. The main Old Testament verses come from the book of Leviticus, where God Himself is speaking. He says, Do not lie with a man as one lies with a woman; that is detestable—God speaking, Leviticus 18:22. This is being said in the context of a number of sexual possibilities that God expressly prohibits. A very similar thought is also spoken by God two chapters later: If a man lies with a man as one lies with a woman, both of them have done what is detestable. They must be put to death; their blood will be on their own heads—God speaking, Leviticus 20:13. This again is brought up in the context of prohibited sexual activity. What is interesting is what God says a few verses later in Leviticus 20: You must not live according to the customs of the nations I am going to drive out before you. Because they did all these things, I abhorred them—God speaking, Leviticus 20:23. A great measure of God’s judgment against the nations Israel found in the Promised Land had to do with prohibited behaviors, specifically sexual prohibited behaviors. It was their ongoing sin that wound up setting God so against them—including sins involving homosexuality. Paul brought this same issue up in the context of the New Testament, writing to the church in Rome, The wrath of God is being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness, since what may be known about God is plain to them, because God has made it plain to them. For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that men are without excuse. For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened. Although they claimed to be wise, they became fools and exchanged the glory of the immortal God for images made to look like mortal man and birds and animals and reptiles. Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. They exchanged the truth about God for a lie, and worshiped and served created things rather than the Creator—who is forever praised. Amen. Because of this, God gave them over to shameful lusts. Even their women exchanged natural relations for unnatural ones. In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion—Paul, Romans 1:18-27. The outcome of this sin, now also specifically addressing and including women, was mentioned by Paul in his first letter to the church in Corinth, a city notorious for it sexual excesses. Paul said, Do you not know that the wicked will not inherit the kingdom of God? Do not be deceived: 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—Paul, I Corinthians 6:9-10. Clearly, Scriptural teachings prohibit any form of same-sex relationship, and nowhere in Scriptures is any kind of same-sex relationship used as a model. Instead, the Scriptures always point to one man being joined to one woman.
But why? Why such a focus on sexual sin? I think it might be because one reason God created marriage was to given humanity a very visible metaphor for the complete and total union God has enjoyed in the Trinity—God the Father in perfect harmony with God the Son and God the Holy Spirit. When man strays outside God’s design, it is one of the most serious affronts that can be committed against Him, because He so personally designed the marriage union to reflect the Trinitarian union. The Bible holds up marriage in an incredibly high platform. When Israel sinned against God by worshiping foreign, false gods, God responded by comparing their act of religious unfaithfulness to the act of marital unfaithfulness between a faithful husband (God) and an unfaithful, wandering wife (Israel). On the positive side, when God is picturing the union of His Son with His church, He sets it in the context of a wedding between an incredible bridegroom (Jesus) and a pure, white bride (the justified church). As the apostle John wrote in the last book of the Bible, Let us rejoice and be glad and give him glory! For the wedding of the Lamb has come, and his bride has made herself ready. Fine linen, bright and clean, was given her to wear.” (Fine linen stands for the righteous acts of the saints.)—Revelation 19:7-8. In fact, Paul used the picture of a bride and bridegroom to illustrate the relationship between Christ and His church: Submit to one another out of reverence for Christ. Wives, submit to your husbands as to the Lord. For the husband is the head of the wife as Christ is the head of the church, his body, of which he is the Savior. Now as the church submits to Christ, so also wives should submit to their husbands in everything. Husbands, love your wives, just as Christ loved the church and gave himself up for her to make her holy, cleansing her by the washing with water through the word, and to present her to himself as a radiant church, without stain or wrinkle or any other blemish, but holy and blameless. In this same way, husbands ought to love their wives as their own bodies. He who loves his wife loves himself. After all, no one ever hated his own body, but he feeds and cares for it, just as Christ does the church—for we are members of his body. “FOR THIS REASON A MAN WILL LEAVE HIS FATHER AND MOTHER AND BE UNITED TO HIS WIFE, AND THE TWO WILL BECOME ONE FLESH.” This is a profound mystery—but I am talking about Christ and the church—Paul, Ephesians 5:21-32 (quoting Genesis 2:24).
So, the question remains: What should we do? It is true that homosexuality is no more a sin issue than many others and, as Paul wonderfully points out to the Corinthian church, is also no more a sin that can be forgiven than any other sin: And that is what some of you were. 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—Paul, I Corinthians 6:11. So my answer is from my own heart, and what I would do. Bear in mind that we live in a world and a context that does not understand the concept of “no.” A prayer is often seen as unanswered, not because God didn’t answer, but because His answer wasn’t “yes.” In the same way, love is now most often defined as that which never says “no.” Well, as a parent, I can tell you that my love and affection for my children often led me to say “no” to them. So, in the same way, friends may not understand how we could love them and still say “no” to them. I think that may be part of the difficulty in this case. But, that being said, I think also that, in the context of being at a same-sex wedding or blessing or whatever, I couldn’t do it because, for me, it would seem to be celebrating that which is clearly sin. I certainly wouldn’t attend a celebration a friend of mine held following the successful robbing of a bank, any more than I would join him in that effort. I think the key is explaining “why” attendance at such an event is not possible—based on your convictions and, more important, based on loving Someone Else much more than your friend. It is to Jesus Christ our allegiance is owed. We also need to remember two other things: (1) We are accompanied by Jesus Christ in the person of the Holy Spirit in all our explanations; as Paul wrote, In the same way, the Spirit helps us in our weakness. We do not know what we ought to pray for, but the Spirit himself intercedes for us with groans that words cannot express. And he who searches our hearts knows the mind of the Spirit, because the Spirit intercedes for the saints in accordance with God’s will—Paul, Romans 8:26-27; and (2) as Jesus said so well, He who listens to you listens to me; he who rejects you rejects me; but he who rejects me rejects him who sent me”—Jesus Christ, Luke 10:16. But nobody wants their salvation greater than the Savior who died to provide it. Ultimately, this is a matter of prayer and then following the leading of the One prayed to.