In the 1st century, a church made up of only Gentiles would not have been aware of the Leviticus passages. If they had received any copies of the letters that were sent to the Corinthians and Timothy they would have been at a loss as to what the Apostle was meaning with his invention of such a word as 'arsenokoites' .
With all due respect, I believe you may be quite mistaken here. In a first century Gentile church, Gentile believers would have immense familiarity with Old Testament Scripture. I have at least two reasons for saying so. First, the New Testament Scriptures make frequent reference to the importance of the Old Testament. Paul taught the Gentile churches about the the immense value of the Scriptures that were already written,
All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, (2 Timothy 3:16)
The early church also valued the public reading of Scripture,
Until I come, devote yourself to the public reading of Scripture, to exhortation, to teaching. (1 Timothy 4:13)
The book of Revelation is replete with allusions and direct references to the Old Testament, and cannot even be understood without the Old Testament Scriptures. This strongly suggests that the first century church would be saturated with the Old Testament.
Because the New Testament Scriptures were not fully complete, and certainly not widely circulated in the first few decades of the church, the Old Testament was the Bible of the early church. It is inconceivable that the Gentiles would thus not be familiar with it. I doubt you will find many historical scholars who disagree.
Well murder is different because in the end you can just not kill the person, but you can't control your sexuality.
Well i think i was asking about both.
Why did he create people knowing they would sin?
And why would he consider homosexuality a sin?
Before we can properly address this issu, it's important to address some of your presuppositions here. You seem to assume a priori that homosexuality is an inherited trait, and that it is sinful to have this trait. Whether homosexuality is inherited or is a matter of free choice is a subject of great debate, but for the purposes of this conversation I am willing to assume that homosexuality is inherited (I shall also assume that the word "marriage" refers to a union between a male and a female, thus excluding homosexual partnerships).
This, however, does not make it right. The Bible teaches that all people, whether heterosexual or homosexual, are born intrinsically sinful. It also teaches that sexual lust and sexual activity outside of marriage are acts of sin. So whether a person is born homosexual or not becomes largely irrelevant. Homosexuals lust after people of the same sex and hetersexuals lust after people of the opposite sex. Both activities are sinful, and sometimes lead to the greater sin of non-marital sex. Homosexuals are not born into any special state of sin. But let us never deny that they are born into sin the same as heterosexuals, and they will go to hell unless they repent of sin and all false gods or religions, and confess Christ Jesus as the Lord.
Now to address your questions. First, why would God create people knowing that they would sin? This is quite a deep question indeed, and no answer I can give will be adequate. The Bible teaches that God is without sin, and he does not create sin. He does, however, ordain that sin should be. He did not create the first two humans without knowing that they would sin and fall from his grace. This, however, was part of his plan to redeem humanity through Jesus Christ. As it says,
He was foreknown before the foundation of the world but was made manifest in the last times for the sake of you who through him are believers in God, who raised him from the dead and gave him glory, so that your faith and hope are in God. (1 Peter 1:20-21)
God seeks to glorify himself, because God is worthy of glory. He could gain glory for himself by creatin people who would serve him in perfect holiness. But he gains greater glory by redeeming sinful people though the death of his very own Son, and creating for himself a church of worshipers who adore him freely and without compulsion. To that end he ordained that sin would enter the world and that people would be born sinful: so that he could display his infinite mercy by shedding the blood of his Son on the Cross and atoning for our sin.
And why does God consider homosexuality a sin specifically? It is a sin because it is a transgression of his covenant of marriage, which he ordained. Marriage does not simply exist so that people can be happy together. Nor does it exist merely as an outlet for sexual pleasure, or for the purpose of procreation. These things are all good consequences of marriage, but marriage exists for a much more profound purpose. Marriage is chiefly a living picture of the relationship between Christ and the church. Christ did not model his relationship with his people after a preexisting marriage institution. On the contrary, God created marriage to model the relationship he intended to have between himself and his people through his Son Jesus Christ. Homosexuality destroys the picture of Christ and his church, and perverts the story that God intended to tell through it. This is why it is wrong.
Ultimately, homosexuality is not the grand sin to end all sins that some Republicans and other political conservatives make it out to be. It's just one symptom of a larger problem. That problem is rejection of Jesus Christ as the blessed and only Sovereign. The world was created by God through Jesus Christ, yet the world rejects Christ as its King. Instead the majority of the world has turned to a plethora of false religions, and a few to atheism. And so they exchange the glory of God for ungodly pleasures, just one of which is homosexuality. All who practice false religion or no religion will go to hell, because Jesus Christ is the only way to salvation. As such, homosexuality should point us to the deeper spiritual problem of open rebellion against the world's Creator. Instead of preaching to you about homosexuality, I would instead admonish you to consider Jesus Christ and his Gospel.