I don't expect everyone to understand that the whole Bible is about Jesus and God's plan to redeem his people, but I vainly hope that one day someone will access their common sense (or at least talk to an informed theological adviser) before leveling the charge of inconsistency.
First, it's not only the Old Testament that has proscriptions about homosexuality. The New Testament has plenty to say about it as well. Even Jesus says, in his discussion of divorce in Matthew 19:3-12, that the original design of God was for one man and one woman to be united as one flesh, and failing that (v. 12), persons should abstain from marriage and sex.
However, let's get back to considering the larger issue of inconsistency regarding things mentioned in the Old Testament no longer practiced by the New Testament people of God. Most Christians don't know what to say when confronted about this issue. Here's a short course on the relationship of the Old Testament to the New Testament.
The Old Testament devotes a good amount of space to describing the various sacrifices offered in the tabernacle (and later temple) to atone for sin so that worshipers could approach a holy God. There was also a complex set of rules for ceremonial purity and cleanness. You could only approach God in worship if you ate certain foods and not others, wore certain forms of dress, refrained from touching a variety of objects, and so on. This vividly conveyed, over and over, that human beings are spiritually unclean and can't go into God's presence without purification.
But even in the Old Testament, many writers hinted that the sacrifices and the temple worship regulations pointed forward to something beyond them (cf. 1 Sam. 15:21-22; Ps. 50:12-15; 51:17; Hos. 6:6). When Christ appeared he declared all foods clean (Mark 7:19), and he ignored the Old Testament cleanliness laws in other ways, touching lepers and dead bodies.
The reason is clear. When he died on the cross the veil in the temple tore, showing that he had done away with the the need for the entire sacrificial system with all its cleanliness laws. Jesus is the ultimate sacrifice for sin, and now Jesus makes us clean.
The entire book of Hebrews explains that the Old Testament ceremonial laws were not so much abolished as fulfilled by Christ. Whenever we pray "in Jesus name" we "have confidence to enter the Most Holy Place by the blood of Jesus" (Heb. 10:19). It would, therefore, be deeply inconsistent with the teaching of the Bible as a whole if we continued to follow the ceremonial laws.
Law Still Binding
The New Testament gives us further guidance about how to read the Old Testament. Paul makes it clear in places like Romans 13:8ff that the apostles understood the Old Testament moral law to still be binding on us. In short, the coming of Christ changed how we worship, but not how we live. The moral law outlines God's own character---his integrity, love, and faithfulness. And so everything the Old Testament says about loving our neighbor, caring for the poor, generosity with our possessions, social relationships, and commitment to our family is still in force. The New Testament continues to forbid killing or committing adultery, and all the sex ethic of the Old Testament is re-stated throughout the New Testament (Matt. 5:27-30; 1 Cor. 6:9-20; 1 Tim. 1:8-11). If the New Testament has reaffirmed a commandment, then it is still in force for us today.
The New Testament explains another change between the testaments. Sins continue to be sins---but the penalties change. In the Old Testament sins like adultery or incest were punishable with civil sanctions like execution. This is because at that time God's people constituted a nation-state, and so all sins had civil penalties.
But in the New Testament the people of God are an assembly of churches all over the world, living under many different governments. The church is not a civil government, and so sins are dealt with by exhortation and, at worst, exclusion from membership. This is how Paul deals with a case of incest in the Corinthian church (1 Cor. 5:1ff. and 2 Cor. 2:7-11). Why this change? Under Christ, the gospel is not confined to a single nation---it has been released to go into all cultures and peoples.
Once you grant the main premise of the Bible---about the surpassing significance of Christ and his salvation---then all the various parts of the Bible make sense. Because of Christ, the ceremonial law is repealed. Because of Christ, the church is no longer a nation-state imposing civil penalties. It all falls into place. However, if you reject the idea of Christ as Son of God and Savior, then, of course, the Bible is at best a mishmash containing some inspiration and wisdom, but most of it would have to be rejected as foolish or erroneous.
So where does this leave us? There are only two possibilities. If Christ is God, then this way of reading the Bible makes sense. The other possibility is that you reject Christianity's basic thesis---you don't believe Jesus is the resurrected Son of God---and then the Bible is no sure guide for you about much of anything. But you can't say in fairness that Christians are being inconsistent with their beliefs to follow the moral statements in the Old Testament while not practicing the other ones.
One way to respond to the charge of inconsistency may be to ask a counter-question: "Are you asking me to deny the very heart of my Christian beliefs?" If you are asked, "Why do you say that?" you could respond, "If I believe Jesus is the resurrected Son of God, I can't follow all the 'clean laws' of diet and practice, and I can't offer animal sacrifices. All that would be to deny the power of Christ's death on the cross. And so those who really believe in Christ must follow some Old Testament texts and not others."
This article originally appeared in Redeemer Presbyterian Church's monthly Redeemer Report.