What does 1 Corinthians 7:5 mean?
Paul continues to reveal God's will about sex within a Christian marriage. He has written that one purpose of marriage is for mutual, unselfish sex to overcome the temptation to participate in sexual immorality outside of marriage.To that end, both husbands and wives have an obligation to the other, as part of their mutual commitment, to be sexually available to each other. The connection between man and woman in a Christian marriage is pictured as being so close and intimate that neither has the authority over his or her own body in this area. That authority belongs to the spouse. This verse phrases this concept as a command, not a suggestion.
Since love is defined by self-sacrifice (Ephesians 5:25), that does not give either spouse justification to "force" themselves on the other. But neither does it justify withholding sex entirely. Both husbands and wives are being commanded, and this service to each other is meant to reflect the self-sacrificing love of Christ. It requires putting the other first in all areas of life. Paul offers no timetables or other details. Ultimately, both parties should be motivated by love and respect for each other.
However, the exception Paul offers here implies an underlying rule: the intention is for sexual union to happen on a regular basis. He allows that, by mutual agreement, sex may be put on hold while husband and wife devote themselves to prayer for a time. This does not mean a person can't be devoted to prayer on a day they engage in sex with a spouse. Instead, Paul seems to be describing a special devotion to prayer, perhaps during a time of fasting.
Once that time is concluded, however, sexual activity should resume. Paul seems to assume that human beings who experience strong sexual desire will struggle with self-control after a while. He also reveals that Satan will take advantage of that struggle to tempt Christians with sexual sin. The closest thing to a rule or schedule Paul offers, then, seems to be that sex between spouses ought to happen with some mutually-agreed frequency.