Although it pertains to the Kingdom of the Left, or God’s left-hand rule (how God orders His creation through His Law), marriage is not an adiaphoron. Severe punishments fall upon those who disregard or disdain God’s institution of marriage–one man and one woman for life. Here’s Philip in the Apology.
Defense of the Augsburg Confession XXIII:55:
Accordingly, at this time, marriage ought to have been especially defended by the most severe laws and warning examples, and men ought to have been invited to marriage. This duty pertains to the magistrates, who ought to maintain public discipline. [God has now so blinded the world that adultery and fornication are permitted almost without punishment; on the contrary, punishment is inflicted on account of marriage. Is not this terrible to hear?] Meanwhile the teachers of the Gospel should do both; they should exhort incontinent men to marriage, and should exhort others not to despise the gift of continence.
