Marriage is to be between one man and one woman. It is not lawful for any man to have more than one wife, nor for any woman to have more than one husband, at the same time.1
1 Gen 2.24; Mal 2.15; Mt 19.5-6
Marriage was ordained for the mutual help of husband and wife,2 for the increase of mankind with legitimate offspring,3 and for preventing immorality.3
2 Gen 2.18 3 Gen 1.28 4 1Cor 7.2, 9
It is lawful for all sorts of people to marry5 if they are able to give their rational consent. However, it is the duty of Christians to marry in the Lord.6 Therefore, those who profess the true religion should not marry with unbelievers or idolaters.7 Neither should those who are godly be unequally yoked by marrying those who are wicked in their life or who maintain [hold and teach] damnable heresy.
5 Heb 13.4; 1Tim 4.3 6 1Cor 7.39 7 Neh 13.25-27
Marriage ought not to be within the degrees of consanguinity [blood relationship] or affinity [close relationship] that are forbidden in the Word.8 Such incestuous marriages can never be made lawful by any law of man or consent of parties in order that those people may live together as man and wife.9