Adam Clarke on Colossians 3:18

Wives, submit yourselves - Having done with general directions, the apostle comes to particular duties, which are commonly called relative; because they only belong to persons in certain situations; and are not incumbent on all. No woman has the duty of a wife to perform but she who is one, and no man has the duty of a husband to perform but he who is married.

The directions here to wives, husbands, children, parents, servants, and masters, are so exactly the same in substance with those in Eph 5:22-33 (note); Eph 6:1-9 (note), that there is no need to repeat what has been said on those passages; and to the notes there the reader is requested to refer.

As it is fit in the Lord - God commands it; and it is both proper and decent.