A.T. Robertson Commentary Hebrews 13:4

"[Let] marriage [be] had in honor among all, and [let] the bed [be] undefiled: for fornicators and adulterers God will judge." — Hebrews 13:4 (ASV)

Let marriage be (ο γαμος). No verb in the Greek. The copula can be supplied either εστιν (is) or εστω (let be, imperative).

Had in honour (τιμιος). Old adjective from τιμη (honour) as in Ac 5:34. Γαμος elsewhere in the N.T., means the wedding or wedding feast (Matthew 22:29; John 2:1).

Undefiled (αμιαντος). Old compound word (alpha privative and verbal of μιαινω, to defile), already in Heb 7:26. Μιαινω την κοιτην is a common expression for adultery.

Fornicators (πορνους). Unmarried and impure.

Adulterers (μοιχους). Impure married persons. God will judge both classes whether men do or not.