Though I find the entire affair absolutely ludicrous, I do agree with this logic. I oppose Christianity at every turn not because they aren't applying their own scriptures evenly but because their scriptures are abhorrent. When Christians bash gays it is not because of Leviticus, but also because of Paul (a raging bigot himself).<quoted text>
Without getting into an exposition of Scripture, the Bible is clear that the Mosaic Covenant (in which the laws you referenced are found) has been replaced by the New Covenant. While there are some similarities between the two covenants, this is not one of them. The Law of Moses was a contract between God and Israel to govern their life in the land of Canaan, both spiritually and politically (a theocratic kingdom). The New Covenant serves an entirely different purpose. The New Covenant governs our spirituality, not our politics. While the New Testament is clear that God has ordained human government to punish evil and commend the good (Romans 13:1-7) it does not speak to how that punishment ought to be carried out for particular moral crimes, or the degree of punishment. It would seem that those decisions are given to the state. It is clear, however, that under the New Covenant sins are not punished in the same way they were under the Mosaic Covenant because the New Covenant is a spiritual, not a political-social covenant.
It would be a mistake, then, to say that Christians are not obeying their own Scripture, or being arbitrary in what parts of the Bible they keep. Each passage of Scripture must be evaluated within its historical and covenantal context, and be applied appropriately. Not all commands in Scripture apply equally for all people and all times (but are specific to a covenant), whereas others do. For example, while I am not obligated to build an ark, I am obligated to abstain from murder.Christians are not ignoring those Old Testament passages, but are properly interpreting them within their context and applying them appropriately.
Yet you do find tons of modern Christians who do in fact quote the Old Testament, especially Leviticus, when it comes to moral questions. That would be the last book I'd look to for moral guidance which is why we give such idiots absolute hell when they do it.