The scriptural arguments that homosexual behaviour is forbidden are quite extensive. I do not intend to address them all at this stage but will touch on an interpretative issue concerning Old Testament texts.
There are prohibitions against sodomy in the Torah (Lev 18:22; 20:13). Some have argued that since several other prohibitions are no longer in effect such as wearing clothes made with blended linen (Deu 22:11), or eating shellfish (Lev 11:10), we can dismiss prohibitions against sodomy. The problem with this kind of approach is that everyone agrees that at least some prohibitions in the Torah are applicable. All would say that people should not murder (Exo 20:13), steal (Exo 20:15), or give false testimony (Exo 20:16). Likewise animal sacrifices are generally agreed to no longer be in effect. Some things mentioned in the Torah are still applicable, and some are not. It is not sufficient to say that sodomy is no longer prohibited because we can eat pork. The question is, "Is the sodomy law akin to laws against theft, or laws against sowing a field with wheat and barley?" If we now sanction sodomy, does the same approval apply to adultery (Exo 20:14), child sacrifice (Lev 18:21), or bestiality (Lev 18:23)?
We gain insight into this question from Paul's letters.
Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither fornicators, nor idolaters, nor adulterers, nor catamites, nor sodomites, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God. And such were some of you. 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)The words relating to sexual immorality above are "fornicator" (πορνοι), "idolater*" (ειδολολατρεσ), "adulterer" (μοιχοι), "catamite" (μαλακοι), and "sodomite" (αρσενοκοιται, arsenokoitai).
Paul elsewhere uses the word "arsenokoites" (αρσενοκοιτεσ) in a vice list in his first letter to Timothy
...the lawless and disobedient, the ungodly and sinners, the unholy and profane, father-killers and mother-killers, murderers, fornicators, sodomites (αρσενοκοιταις), kidnappers, liars, perjurers, and whatever else is contrary to sound doctrine, (1 Timothy 1:9-10)This is a compound word that some claim Paul coined. It is derived from "arsen" (αρσεν) meaning "male" and "koite" (κοιτε) meaning "bed". That "bed" (koite) can have sexual overlay in the meaning (compare to, "bed a girl") can be seen in that we get "coitus" from this Greek word. This word is a reference back to Leviticus
καὶ ὃς ἂν κοιμηθῇ μετὰ ἄρσενος κοίτην γυναικός βδέλυγμα ἐποίησαν ἀμφότεροι θανατούσθωσαν ἔνοχοί εἰσιν (Lev 20:3, Greek Septuagint)Paul's use of this word in his lists is straight from Leviticus which favours reading the ruling in Leviticus as a permanent prohibition and not a temporary injunction for the Israelites.
And he who lies with a male in a bed as with a woman, both of them have committed an abomination; let them certainly be put to death, they are guilty. (Lev 20:13, English Septuagint)
I think both the design of human anatomy and the commands of God provide us with the answer to homosexual practice. Nevertheless, an understanding of the broader aspects of sexuality and God's intention can help us understand not just what God commands but why.
*While idolatry is not intrinsically sexual, in practice it is often closely associated with sexual immorality.