And the man and his wife were both naked and were not ashamed. (Genesis 2:25)
Sinless and unbroken, Adam and Eve had a beautiful innocence about them. They were both naked and were not ashamed (25). The implication is that shame would come due to the fall, but Adam and Eve didn't have it. They were to enjoy each other in every way, including sexually.
And, as much as the comment might shock us, it would have especially jarred ancient Israel. They were part of a much more bashful society, and the rest of the Old Testament rebukes various forms of nakedness. But, in the garden, in their blissful married state, God put His seal of approval on their nakedness.
The common view in our world is that Christianity is repressive and anti-sex, but, though Christians might adopt anti-biblical views on sex, the Bible is thoroughly pro-sex. Consider the following verses:
Proverbs 5:18–19 (ESV) — 18 Let your fountain be blessed, and rejoice in the wife of your youth, 19 a lovely deer, a graceful doe. Let her breasts fill you at all times with delight; be intoxicated always in her love.
Song of Solomon 1:2 (ESV) — 2 Let him kiss me with the kisses of his mouth! For your love is better than wine;
Song of Solomon 4:6 (ESV) — 6 Until the day breathes and the shadows flee, I will go away to the mountain of myrrh and the hill of frankincense.
Modern translators dance around 1 Corinthians 7:3 a little bit, using phrases like marital responsibility, duty, due affection, and the giving of conjugal rights. The NLT gets down to it:
1 Corinthians 7:3 (NLT) — 3 The husband should fulfill his wife’s sexual needs, and the wife should fulfill her husband’s needs.
So Scripture is clear. God is interested in promoting the sexual life of heterosexual married couples. To Him, sex inside marriage is beautiful. It was part of His original and untainted creation (24).
One reason it's beautiful inside marriage, but not outside it, is that inside marriage it is intended to be safe, build trust, serve someone else, lead to a family, and be marital glue. Because of sin, it might not be these things inside a particular marriage, but outside marriage, it is never these things.
If you are married, do not invite impurity into your marriage. Don't allow lust or pornography or abuse or neglect or atrophy into the marriage bed. The fire of sexual love belongs in the fireplace of marital commitment. If put in any other place, the fire harms, but in its proper context, it warms and helps.