1 Corinthians 7:4 The wife hath not power of her own body, but the husband: and likewise also the husband hath not power of his own body, but the wife.
5 Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.
Neither of those statements say that a wife sins by not sleeping with her husband, except by your inference, your personal, individual, flawed, interpretation. In fact neither statement even talks about sex ...
So ... what do you think that these passages spek of ... ?
well, I read the first as saying that man and women need to work as a team and the second as dealing with prayer.