I've been reading John Piper's chapters in 'Sex and the supremacy of Christ' and finding them less irritatingly American than some of his stuff. Here's his basic thesis:

'sexuality is designed by God as a way to know God in Christ more fully.'
AND 'knowing God in Christ more fully is designed as a way of guarding and guiding our sexuality.'

Piper restates his point negatively:

'all sexual corruption serves to conceal the true knowledge of Christ, but the true knowledge of Christ serves to prevent sexual corruption.'

I guess we'd all be happy to point to the sexual depravity of society as evidence of the rejection of Christ (and as Romans 1 reminds us, a judgment on such rejection). But some would find Piper's first point more controversial. He is suggesting that somehow human sexuality aids our knowledge of God.

I think it's amazing how many people claim (apparently without feeling the need to justify this in any way) that there is nothing in the bible to suggest that our relationship with God could be thought of in sexual terms. Piper rightly refutes this, citing Ezekiel 16:4-10 in which God speaks of Judah as his bride over whom he laid his garment, a clear sexual reference. This is confirmed when Judah's unfaithfulness is described in sexual terms - as prostitution.

Piper again:
'sexual language in the Bible for our covenant relationship to God does lead us to think of knowing God on the analogy of sexual intimacy and ecstasy. I don't mean that we somehow have sexual relations with God or he with man. ...But I do mean that the intimacy and ecstasy of sexual relations points to what knowing God is meant to be.'

Quite right. Odd then, that other contributors to the book (Ben Patterson, CJ Mahaney) are so certain that the Song of Songs is a book only about human sexuality. Since sexual language is used elsewhere of God's relationship to his people, why not here?