“It is lawful for all sorts of people to marry…” ( XXIV. 3).
Since the WCF has stated elsewhere (XVI.7) that “works done by unregenerate men” are “sinful, and cannot please God” then the question is what exactly do they mean when they say that it is “lawful for all sorts of people to marry”? Their citation of Hebrews 13:4 for this assertion in XXIV.3 might imply that they believed that the marriage of unregenerate men is NOT sinful fornication, but rather honorable and undefiled marriage.
But it could also imply that the men of Westminster Abbey believed that the marriage of unregenerate persons is somehow lawful and yet also sinful. If they did mean to convey this, it would have helped to not cite Hebrews 13:4 for those whose works are “sinful, and cannot please God.”
The marriage of an unregenerate virgin man and virgin woman, while a legitimate marriage in the Genesis 2:24, Matthew 19:4-9, 1 Corinthians 6:16-18 sense it is obviously NOT lawful or honorable since there are whoremongers (fornicators) and adulterers whom God will judge.
In 1 Corinthians 6 Paul quotes Genesis 2:24 to make the crucial point that the one who joins himself sexually to a harlot has formed a legitimate–but obviously NOT lawful per Hebrews 13:4–marriage with the harlot: “for two, saith he, shall be one flesh” (1 Corinthians 6:16). This legitimate marriage is by no means a lawful or honorable marriage, which is why the Apostle commands that it must be fled from (1 Corinthians 6:18).
Hebrews 11:6 says that without faith it is impossible to please God, all unregenerate persons are without faith, thus their marriage while legitimate in the aforementioned sense, is not pleasing to God as a lawful marriage. Nothing they do pleases God and therefore nothing they do or engage in is lawful.
This is NOT to say that Christians are forbidding unregenerate persons to marry (1 Timothy 4:3) any more than they are forbidding them to plow their fields (Proverbs 21:4) or to provide for their families (1 Timothy 5:8); but it is to say that just as “the plowing of the wicked is sin” (Proverbs 21:4), so is everything else they do (Hebrews 11:6). All of their ignorant zeal (Romans 10:3) and all of their sincere attempts at religion and morality are an abomination to God (Proverbs 21:27).