1689 London Baptist Confession of Faith--Chapter 25: Of Marriage

1._____ Marriage is to be between one man and one woman; neither is it lawful for any man to have more than one wife, nor for any woman to have more than one husband at the same time.
( Genesis 2:24; Malachi 2:15; Matthew 19:5,6 )

2._____ Marriage was ordained for the mutual help of husband and wife, for the increase of mankind with a legitimate issue, and the preventing of uncleanness.
( Genesis 2:18; Genesis 1:28; 1 Corinthians 7:2, 9 )

3._____ It is lawful for all sorts of people to marry, who are able with judgment to give their consent; yet it is the duty of Christians to marry in the Lord; and therefore such as profess the true religion, should not marry with infidels, or idolaters; neither should such as are godly, be unequally yoked, by marrying with such as are wicked in their life, or maintain damnable heresy.
( Hebrews 13:4; 1 Timothy 4:3; 1 Corinthians 7:39; Nehemiah 13:25-27 )

4._____ Marriage ought not to be within the degrees of consanguinity or affinity, forbidden in the Word; nor can such incestuous marriages ever be made lawful, by any law of man or consent of parties, so as those persons may live together as man and wife.
( Leviticus 18; Mark 6:18; 1 Corinthians 5:1 )