It's better to look at sex and the body as a whole theme in Scripture. Here are some themes and verses that back them.
A couple of things to begin with:
- Let’s talk about "sex outside of marriage" (you can be "married" and have sex - with someone else! - that's not intended by God.) Pre-marital sex is just sex outside of marriage.
- I like to think in terms of, God's Design for Sex, What the Bible Has to Say About Sex, Sex in all of its Beauty. This way, we aren’t focusing on what we can't do (and how far we can push the envelope without crossing the line), but instead focusing on what to do. That's what the Bible wholly talks about - the life we are called to live, rather than the one that we're not called to live.
Bible and Sex:
I see the purpose of sex, as outlined in Scripture, as the central intimate act for a man and women. Emotionally, physically, spiritually, it's the unique, intimate act between a man and woman. It's a coming together on all of those levels.
Man and women coming together in marriage is a big deal:
Matthew 19.1-6 - in response to a question about separation, Jesus says it's not that easy. In marriage (and sex), the man and woman are bound together as one being. Just as you can't separate your own body into two parts. Put sex into this mix - the most intimate act emotionally, physically, spiritually - and it gets really messy. Trying to separate from each of those "joining togethers" is, it seems according to the text, a myth.
Our bodies don't belong to us:
1 Corinthians 6.12-20 - our body belongs to God. He purchased us with the death of Jesus. And how we use our bodies’ matters to God, and is an opportunity to reveal His greatness. Sexual immorality is a sin against this body and the intention that we glorify Him (display His greatness in how we use this body).
Sexual purity is God's will for us:
1 Thess. 4.3-8 - one of the few places in the entire Bible (1 Thess 5.18 is another) where "God's will" is stated in those words. Our body is to be used for honor, and intended for sex in all of its beauty. Sexual purity mean keeping sex in its context and design (marriage).
Our bodies are a living act of worship to God:
Romans 12.1-2 - again, our bodies aren't for our purposes. Our bodies are a place and instrument of worship to God. How we use our bodies matters to God, and it matters to show God in the world. Even if the world doesn't give its bodies to the glory of God, Paul challenges to not be conformed to the way the world operates, but be transformed.
Here also is a section in the commentary, "Bible Knowledge Commentary" on the book of Song of Songs:
"The purpose of the book is to extol human love and marriage. Though at first this seems strange, on reflection it is not surprising for God to have included in the biblical canon a book endorsing the beauty and purity of marital love. God created man and woman (Gen. 1:27; 2:20-23) and established and sanctioned marriage (Gen. 2:24). Since the world views sex so sordidly and perverts and exploits it so persistently and since so many marriages are crumbling because of lack of love, commitment, and devotion, it is advantageous to have a book in the Bible that gives God’s endorsement of marital love as wholesome and pure."
On the following podcast (Sex and the Christian Hope), Tim Keller does say that the Greek word (in 1 Cor 6) for "sexual immorality" means having sex outside of marriage. About 8 mins in to the podcast.
God created sex. He's all for sex. There's no one more in favor of sex than God - but in the context (marriage) and purpose (glory) that He designed. That, I think, is sex in all its beauty.