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 (KJV)

Then: Paul's extension from verse 3 clarifies and succinctly limits the marriage relationship to one man and one woman. Only one husband can have power over the wife's body and only one wife can have power over the husband's. There is no allowance for a third party in either instance. Henry concentrates on the idea of keeping human vessels in a state of "sanctifcation and honor." Abiding by conjugal duty is therefore Paul's primary advice. That avoids incentives for temptations - especially in a city as cosmopolitan as Corinth (Wright, p. 78).

Now: The understanding from earlier translations has changed from an emphasis on marriage because of sexual needs to a more positive view. The body is the temple of the Lord, and all aspects of good marital relationships can preclude the danger of slipping into immorality (Wright, p. 78).

Extra source: Commentary on the Whole Bible (Acts to Revelation) by Matthew Henry, online see http://www.ccel.org/ccel/henry/mhc6.iCor.viii.html