Jamieson, Fausset and Brown on 1 Corinthians 7:5

5. Defraud ... not--namely, of the conjugal duty "due" (1Co 7:3; compare the Septuagint, Ex 21:10).

except it be--"unless perchance" [Alford].

give yourselves to--literally, "be at leisure for"; be free from interruptions for; namely, on some special "season," as the Greek for "time" means (compare Ex 19:15; Joe 2:16; Zec 7:3).

fasting and prayer--The oldest manuscripts omit "fasting and"; an interpolation, evidently, of ascetics.

come together--The oldest manuscripts read, "be together," namely, in the regular state of the married.

Satan--who often thrusts in his temptations to unholy thoughts amidst the holiest exercises.

for your incontinency--because of your inability to "contain" (1Co 7:9) your natural propensities, which Satan would take advantage of.