Scripture nowhere teaches that sex for enjoyment is wrong within a marriage as long as sin is not otherwise being committed. In fact, married couples are specifically commanded to have sex.
First, though, we should look at why God created marriage.
Eph 5:31-32 (NIV) "For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh." This is a profound mystery--but I am talking about Christ and the church...Note that the purpose of marriage is defined for us here, and no mention of kids. Children are indeed a hope and blessing for many, but not God's primary purpose for marriage. God made marriage to show us a "great mystery" (KJV)... of the joy and bliss of communion between God and man. It is not the only purpose, but the principal, and the one God says was His motive for creating sexual capability.
According to Scripture, sex within marriage is the "God ordained sermon" given us to understand how it is and will be with Him. "A taste of heaven" is a nice paraphrase of God's intent. Been to church lately and heard this sermon? It is God's purest and clearest.
Elsewhere, married couples are commanded not to abstain from "normal sexual relations" (the Phillips of below)...
1Cor 7:5 (NIV) Do not deprive each other except by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control.Notice the exclusive case God allows for the sexual desire within marriage not to be consummated is only for a "fast" for the purpose of prayer. It is not in any way related to having or not having children.
Again, in the Old Testament:
Prov 5:18-19 (NAS) May your fountain be blessed, and may you rejoice in the wife of your youth. A loving doe, a graceful deer--may her breasts satisfy you always, may you ever be captivated by her love.Note that in first part of this verse there is the connotation of children being anticipated as a blessing, for "fountain" is a metaphor (directly in Essene writings) of male ejaculation. And this is representative of many Scriptures.
Prov 5:18-19 (Jer) ...Find joy with the wife you married in your youth... hers the breasts that ever fill you with delight, hers the love that ever holds you captive.
But more to our point here, "enjoy the breasts of your wife" is an on-going thing. A baby, if the couple and the Lord will it, is a once a year (or every few years) thing. But we are to enjoy each other in God's blessing and grace.
Notice also the goal, that "you ever be captivated by her love". See the connection to 1 Cor 7:5? Paul turns this advice into a command "lest you be tempted" due to lack of self control.
I hope this makes sufficiently clear that God made sex within marriage to be enjoyed and not just for the occasional kid.
All that said, the purity of the marriage bed can be defiled before going in, and/or sullied by perverted fantasies and kinky practices within. Augustine and others have written of "the sin of lust" within a marriage, and when I was single this made zero sense to me. But now that I am married it makes perfect sense. It is definitely possible to have sinful sex within a marriage. So what is the difference between sinful lust as opposed to normal sexual desire in marriage?
In short, married couples sin when extra-marital sexual fantasies enter into the marriage bed. Such mental adultery often comes as a result of trying to maintain an overactive sex-life. Take, for example, role-playing: unless your husband or wife is who they are acting like or dressing like in such fantasy sex, isn't this a rather obvious outlet for lust to be with someone else, and an insult to the sufficiency of God's provision? Such we are convinced is sin, as it clearly has nothing to do with the intent of God to communicate to us the glory of the communion between Christ and the Church through sex. Further, whenever private adulterous thoughts are brought in to "improve" sex, the marriage bed has become adulterous even if the couple having sex is married. This is why pornography is so damaging to marriage: if a man is thinking of another woman while making love to his wife, the marriage bed is spiritually defiled with adultery, and sexual problems will inevitably surface as this hypocrisy sows death and destruction.
As a man, I am very careful not to allow these lusty elements into my mind or marriage to create a "heightened", inordinate sexual urge or experience. For once they are let in, they are not so easy to get rid of. Instead, I revel in what God so richly provided without playing the world's lust games.
But this itself was a work of grace and deliverance. While both Laura and I were virgins before marriage, I was a full-tilt pervert. "If you have lusted in your mind..." and boy had I. I also had a problem with pornography which fed the pent-up lust I thought was normal sexuality. So I was a mess. The Lord led us to seek prayer and deliverance before marriage, with the result that God completely took away any and all sexual urge or instinct in me. I was made a eunuch. Very strange, but true.
At first, I was not sure I wanted my prayers answered in this way, but it was better than burning in lust. As such, and with Laura (well) aware of the situation, we got engaged and later married--all without any sexual attraction on my part. I could have passed the "postage stamp" test for months on end--I had no sexual desire or function whatsoever and not even the spark of anything like it. The question was, of course, would I ever be able to sexually function again? For I was not trying to do this, in fact I was rather in disbelief it was happening. But it was, and completely and totally, and I was going to get married! This was an act of faith and very risky, but we both believed it was God's will.
It tuned out this was the greatest wedding present God could have given us. After we were married, a new thing resurrected in place of the perverted lust I thought was sexuality before. This new thing is pure and measured and loving and very pleasing and satisfying, almost the opposite of everything that I thought was sex before. As soon as I sense that pull of the old lure, I am careful to seek prayer from Laura and others lest I fall into cancerous lust again.
My old sexuality had to be killed, and it was so perverted after all that was evil in it was put on the Cross that nothing was left afterwards. But God, as is His nature and promise, gave me a new life in this area--scary as it was in the interim--and for this we praise Him. The old could not be salvaged; it had to be crucified. This is my story and experience, for what it is worth.
God made sex. And He made it for a purpose. Part of us (the pervert) must die in order for God's purpose to be resurrected in the impure.
So we would not say that it is impossible to sin within marriage. The scriptural support for this is that God looks on the heart, and if we are projecting on to our wives images outside of the marriage, or practices we learned from some illicit source or other person or pornography, can our marriage bed be said to be pure?
Heb 13:4 (NIV) Marriage should be honoured by all, and the marriage bed kept pure...Let us not be bound by tradition or superstition or false teaching concerning God's providence and "sermon of sex" in marriage, nor allow any sin to poison the ordinate use of this gift of God.