I think the fundamental problem of those you're talking with on the other forum is that, since they (seem to) view the NT as a replacement for the OT (instead of the fulfillment/continuation of it, as Jesus said), they therefore consider God's law capable of changing on a whim. But God's law is an expression of his being and can't change regardless of the covenant he condescends to make with men. When Paul says that adultery is wrong, that is law. In another place he says Christian are "not under law"; but what he means there is that Christians are not under the law as a covenant of works. If you absolutize the statement that we are "not under law," then you make Paul contradict himself when he goes on to tell Christians what sins they ought not to be committing (which, again, is law)!

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Casey Bessette
Westminster OPC • West Suburbs of Chicago • My Blog: Paradise Regained

"It is part of the calling of the ekklesia to learn to know the love of Christ that surpasses all knowledge and also to make known within the world of science 'the manifold wisdom of God' in order that the final end of theology, as of all things, may be that the name of the Lord is glorified. Theology and dogmatics, too, exist for the Lord's sake." — Herman Bavinck, Reformed Dogmatics, vol. 1, p. 46