If a woman denies sex to her husband, is this unlawful sex and, therefore, fornication? If a twenty year old wife refuses to have sex with her twenty-one year old husband, does he have any grounds to divorce?
As stated in a prior answer, when a person wants to do something, such as divorce their spouse in order to marry someone else, but they can't find scriptural justification for their action, they will either try to dismiss the restriction by saying the command is not relevant today or they will try to broaden the restriction so that what they want is included in the command. In other words, they will either try to subtract from God's word or they will try to add to it.
In this Greek word porneia is trying to be expanded to include refusal to have sex. Of course this would not work. Porneia is used to describe illicit sexual acts, including having sex outside the covenant of marriage. Those an unmarried couple having sex is properly called porneia or fornication. But if the definition was expanded to include refusing to have sex, then a girl who refuses a boy's sexual advances would be charged with fornication! She would be condemned if she did and condemned if she didn't. Now isn't that a wonderful situation.
The fact is that porneia only refers to illicit sexual acts, not the lack of such acts. Yes, this wife is sinning by refusing to have sex with her husband -- not because it is porneia, but because the Bible says, "Let the husband render to his wife the affection due her, and likewise also the wife to her husband. The wife does not have authority over her own body, but the husband does. And likewise the husband does not have authority over his own body, but the wife does. Do not deprive one another except with consent for a time, that you may give yourselves to fasting and prayer; and come together again so that Satan does not tempt you because of your lack of self-control" (I Corinthians 7:3-5).
A husband might chose to divorce his wife because of this (though he should not), but he would have to remain unmarried. In other words, his divorce will not change the situation. "Now to the married I command, yet not I but the Lord: A wife is not to depart from her husband. But even if she does depart, let her remain unmarried or be reconciled to her husband. And a husband is not to divorce his wife" (I Corinthians 7:10-11). The problem is that this talk of divorce is distracting from the problem. The question is: Why would a wife risk the eternal condemnation of God because she doesn't want to fulfill her husband's sexual desire?
I suspect that there real reasons behind her rejection of her husband that she would rather not say since sex is such an intimate act. However, these issues need to be addressed because it is destroying their marriage. It may be that she has lost respect for her husband because of what he has done. It may be that she has something physically wrong that is making sex painful. Or, it might be that her husband never learned how to properly have sex and is causing her discomfort by having intercourse too quickly or embarrassing her by his demands of how sex is done. There is no way to know until she is willing to talk about what the problem is. And there is no way to fix it until both she and he are willing to make changes in their lives.
I know in several different cases that the wife was just passing time. And why no sex? She had planned a future divorce at the right time. She was actually having sex with someone else and that was why she was not having sex with her husband.
Such can also be a possibility. While unfaithfulness is a ground for divorce that allows for the innocent party to remarry, the lack of sex alone is not proof that adultery is taking place.