In 1st Corinthians 7, the Apostle Paul encourages spouses to give of themselves sexually to their spouse. Contrary to what many think, the Bible does not support a platonic view human sexuality; it is very much pro-sex. However, Paul does state that sexual immorality should be resisted and abstained from.
1 Corinthians 6:18
Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body.
Now, the word he uses for sexual immorality is “porneia” which was a word with a broad meaning. (See Mt 5:32; 15:19; 19:9; Mk 7:21; Jn 8:41; Ac 15:20; 1Co 6:18; 7:2; 2Co 12:21; Gal 5:19; Eph 5:3; 1Th 4:3.) It denotes any kind of illegitimate sexual intercourse or relationship and it is typically translated in the NIV as “sexual immorality.”
In this particular passage “sexual immorality” is most definitely referring to any sexual act outside of the parameters of a marriage between a husband and a wife (see 1 Cor. 6:12 – 7:7) and we are told to “flee” from it – to have nothing to do with it! Why? Because of the unifying/uniting power of sexual intercourse. In verse 6:16, he states a very profound reason way people should abstain from sex outside of marriage - “Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, ‘The two will become one flesh.’ I once heard Tim Keller refer to sex as a “unitive act….It is a way of cementing and enabling relationships of complete oneness... It is God's appointed way to say to another person, I belong completely and exclusively to you." And, for those who have had sex outside of marriage, you know this is true because that's how you felt when you first had sex; you felt such a strong (premature) connection between the both of you, almost like you would if you were married. Premarital and/or extra-marital sex violates the intrinsic meaning of sex.
The problem with sex outside of marriage is that it performs “a life-uniting act without a life-uniting intent,” and thus you violate the intrinsic meaning of sex. Sex is a way to say I am completely yours; but if you are having “porneia” (sex outside of marriage) you are using sex to say something else, namely, “I don't love you enough to marry you, because if I marry you, I put myself at your mercy, what's wrong in your life become wrong in mine.” Commitment like this is really scary to most of us which is why we live in a culture that wants the freedom of having sex without any consequences. This is why we live with our boyfriends, why we use condoms, or have “friends with benefits.” But, like it or not, there is no such thing as “porneia” without consequences because of how God has made us. Sex only works with total openness and vulnerability. Tim Keller once said, “physical nakedness without total nakedness violates the meaning of sex.”
Marriage is to be a human covenant; and sex in marriage is a covenant renewal ceremony in which you say "I am completely yours." The Bible often uses the Hebrew word "yada" (which is often translated “to know”) for sexual relations and intercourse in marriage, because sex is about relational and emotional oneness not just physical oneness or penetration. (FYI - This is why most sexual problems in marriage are really relationship problems at their root.)
So, this brings us to heart of the question at hand, can sex within marriage ever be wrong? Well, if sex between husband isn’t the giving of themselves to each other that Paul talks about is 1st Corinthians 7, it is being approached all wrong. Speaking about sex in marriage, Paul says, “The husband should fulfill his marital duty to his wife, and likewise the wife to her husband. The wife’s body does not belong to her alone but also to her husband. In the same way, the husband’s body does not belong to him alone but also to his wife. Do not deprive each other….” In sex, we are to give of our self to our spouse and focus on their pleasure - not just our own gratification. In other words, sex is to be a celebration of the marital union of relational, emotional, and physical oneness; ultimately it is to be an act of self-donation, not self-gratification. (FYI - This is why masturbation is fundamentally wrong; it focuses only on self.) And speaking of masturbation, couples who have sex without trying to pleasure their spouse are basically participating in mutual-masturbation. And ultimately, if this is not addressed, sex will actually drive a wedge of disappointment and hurt between the two spouses instead of unifying them.