Let marriage be (ho gamos). No verb in the Greek. The copula can be supplied either estin (is) or estō (let be, imperative).
Had in honour (timios). Old adjective from timē (honour) as in Act 5:34. Gamos elsewhere in the N.T., means the wedding or wedding feast (Mat 22:29; Joh 2:1).
Undefiled (amiantos). Old compound word (alpha privative and verbal of miainō, to defile), already in Heb 7:26. Miainō tēn koitēn is a common expression for adultery.
Fornicators (pornous). Unmarried and impure.
Adulterers (moichous). Impure married persons. God will judge both classes whether men do or not.