Institutes of the Christian Religion (1559) - John Calvin
Book 2 - God the Redeemer
Chapter 8 - Explanation of the Moral Law:
The 10 Commandments
Sections 41-44 - Seventh Commandment - No Adultery
Section 41 - Interpretation
Forbids all "lustful intemperance of the flesh."
"Any other union apart from marriage is accursed in His sight."
Section 42 - Celibacy?
By our nature and the Fall, men are "doubly subject to women's society."
Men should lean toward marriage, rather than "strive foolishly and rashly to overcome and surmount their necessities."
If God hasn't given it, don't seek it - Matt 19:11-12; 1 Cor 7:7.
Section 43 - Marriage as related to this commandment
Celibacy can't be achieved, but is granted by God.
It isn't enough to control your outward behavior if your "heart inwardly burns with lust."
"It is better to marry than to burn" - 1 Cor 7:9.
Section 44 - Modesty and chastity
The sexual relationship is good, but ought not be polluted by "uncontrolled and dissolute lust."
So, married couples can't do whatever they want, sexually.
Modesty also forbids "us to seduce the modesty of another with wanton dress and obscene gestures and foul speech."