Homosexuality.
God has said much about this sin.
Firstly, and most famously, homosexuality is the only sin for which God rained fire and sulfur down from heaven to destroy entire cities. (Genesis 13:13, 19:4-10, 19:24-25, Jude 7)
Thou shalt not lie with mankind, as with womankind: it is abomination. (Leviticus 18:22)
If a man also lie with mankind, as he lieth with a woman, both of them have committed an abomination: (Leviticus 20:13)
Here is a lengthy excerpt from Romans (1: 18-32) that explains how a people who knew God refused to glorify him, and instead trusted in their own "wisdom." When the people refused to acknowledge God, he left them to themselves, and immediately they turned to homosexual perversions, choosing to do unnatural things.
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness; Because that which may be known of God is manifest in them; for God hath shewed it unto them. For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. Professing themselves to be wise, they became fools, And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things. Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves: Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen. For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature: And likewise also the men, leaving the natural use of the woman, burned in their lust one toward another; men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet. And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient; Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents, Without understanding, covenantbreakers, without natural affection, implacable, unmerciful: Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them.
In 1 Corinthians 6: 9-11, God lists "abusers of themselves with mankind" (Sodomites) among those unrighteous who cannot inherit his kingdom. (This is a broad list of sins, the purpose of which is to prove that no one is worthy of God's kingdom.) Then, God points out that people were washed, sanctified, justified in Jesus' name... so clearly these homosexuals weren't born as such, they were choosing to sin against God. And they were able to stop making that choice too.
Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God. And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God.
The same idea is repeated in 1 Timothy 1:10.
Knowing this, that the law is not made for a righteous man, but for the lawless and disobedient, for the ungodly and for sinners, for unholy and profane, for murderers of fathers and murderers of mothers, for manslayers, For whoremongers, for them that defile themselves with mankind, for menstealers, for liars, for perjured persons, and if there be any other thing that is contrary to sound doctrine;
All of Judges 19 tells of a mob of homosexual men who attempt to perform their evil desires on a visitor to the city. When the visitor's host refuses their demand, the mob rapes and murders a concubine.
So what is the answer? What are we to do as Christians? We are to love all sinners (which is, everyone), and point out their sins so that they may repent and trust Christ. Acceptance of sin is not love. If you see someone walking down a path that leads to destruction, the only truly loving thing you can do is show them the error of their ways. Explain that God has rules by which he will judge us. Explain that God is not willing that any should perish, but that all would turn to him. Explain that they need to choose whether to follow the lusts of their own heart, or follow God. These things apply to all sinners including homosexuals.