‘Do not defraud one the other, unless it is by consent for a time, that you my give yourselves to prayer, and may be together again, so that Satan does not tempt you because of your lack of self-control.'
Indeed to refuse intercourse and proper lovemaking without good reason is to defraud one's partner, and therefore sinful. It is failing to recognise their justifiable needs, and to recognise those needs is man's duty (1 Corinthians 7:3). An exception may be made, by agreement, for a short break, for the purposes of a time for prayer and spiritual advancement, but this should not be overlong and they should then come together again after a reasonable time in case Satan gets the opportunity to tempt them due to their inability to control their desires.
Notice here that the abstinence is not because sexual relations within marriage are somehow ‘sinful' but simply in order to concentrate more on the particular spiritual activity in mind.
Satan is here seen as the shadowy background figure who will take any opportunity to cause men to fall.