William Burkitt on 1 Corinthians 7:3

Observe here, 1. That matrimonial conversation, or the husband's and wife's performing towards each other all the duties of marriage which they promised, is an act of justice, which they owe to one another: this is intimated in the word render, and consequently to deny the same is injustice and fraud: Defraud not one another. Marriage takes away from persons that power which they had over themselves and their own bodies, and transfers it in some sort to the person they are married to.

Yet observe, 2. That persons in a married state may, and in some cases ought, (namely, for religious ends and purposes,) by mutual consent to abstain from a conjugal duty for some time: Defraud not one another, except it be with consent for a time, that ye may give yourselves to fasting and prayer.

Observe, 3. The apostle lays no obligation upon any single persons to take upon them a vow for a single life, nor doth he direct married persons to those perpetual divorces from the marriage-bed, which the papists practise, under pretence of religion: for the apostle admits of no perpetual separation between husband and wife, upon any pretence whatever: no, not that they may give themselves to prayer and fasting; but only permits it for a time, upon condition that they come together again. So far was this holy man from laying a snare upon the consciences of any persons, either in a single or married state.