In that view, I find human nature to be a notion entirely pitted against. There is a balance that is required. It is natural to marry, to love, to procreate, etc. Anyone who advises otherwise, is not from the religion of surrendering one's entire self to God. The very marriage act and subsequent relationship is a sanctified and holy endeavor that it strongly advise in Islam.
How do we know which one is "correct"? In Islamic teaching, we have been given this as knowledge from our Lord, through God's Messenger. That alone is sufficient for us. For disbelievers, there is still considerable persuasive power in this line of thinking. Human beings have definite limits and needs. A little divine help in this area would sure be appreciated. We do have "social contracts" and "inner demons". Both of these are only properly dealt with when we have the right understanding of human action and responsbility. Our knowledge of this can best be delivered to us by God, because we do not have the capacity to forsee our errors, except by conjecture and experimentation. Even then, the criteria of success is impossible to define intersubjectively to everyone's favour and delight.
God along is sufficient to describe and select the boundaries for His Creation.
As for the associative identities of groups people and the path towards purification...no definition can exhaustively define a person and their relation to the Divine. Having one's identity bound up with associating to God on intersubjective grounds as in the case of a Christian (vis-a-vis Jesus) doesn't cause fluidity in legal adherence, it completely dissolves the barriers of how to define right action.
We are absolutely compelled to make judgment calls, no matter what. When Jesus is made a man and God, we give up the ability to accept judgment calls from God.
In this type of Christianity that you describe, one gives up the ability to discernibly judge the theological basis of Christ, because it is completely bound up in the subjective relationships to the experience of Christ. I don't find any right for a Christian to say that anyone else's relationship towards Jesus is faulty on theological grounds - but they may find vehement reasons to be personally put-off by others.
The idea that, " it's insufficient to be "good" according to the old Law or any other set of rules" is hyperbole. It means that no one is perfect like God is perfect. Our actions are limited and our consciousness of God is screened by a veil. One may, however, certainly find that the Law is a path to Righteousness because:
The entire cosmology of Christianity can be replicated in Islamic terms, except for the Divinity of Christ. The balance between spiritual development, social reform, and Justice has all of the same imagery. The main difference is that the imagination in Christian terms has been a path of self-discovery through sinful regression and remorseful redemption. This is viewed as the very path of the Christian, dying to Christ. However, all of this struggle can be directed through the Islamic prism also. When it is channels through that path, the struggle is a guided one. The guidance of God is the best of guidance.James 1: 13-15, 19-25 wrote: ...no one should say, “God is tempting me.” For God cannot be tempted by evil, nor does he tempt anyone; 14 but each person is tempted when they are dragged away by their own evil desire and enticed. 15 Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death...
... 19 My dear brothers and sisters, take note of this: Everyone should be quick to listen, slow to speak and slow to become angry, 20 because human anger does not produce the righteousness that God desires. 21 Therefore, get rid of all moral filth and the evil that is so prevalent and humbly accept the word planted in you, which can save you. 22 Do not merely listen to the word, and so deceive yourselves. Do what it says. 23 Anyone who listens to the word but does not do what it says is like someone who looks at his face in a mirror 24 and, after looking at himself, goes away and immediately forgets what he looks like. 25 But whoever looks intently into the perfect law that gives freedom, and continues in it—not forgetting what they have heard, but doing it—they will be blessed in what they do.
Peace,
-sgttomas