Marriage is to be between one man and one woman. A man must not have more than one wife nor a woman more than one husband at the same time (Ge 2:24; Mal 2:15; Mt 19:5-6).
Marriage was ordained for the mutual help of husband and wife (Ge 2:18), for the increase of humanity with legitimate offspring (Ge 1:28), and for the prevention of immorality (1Co 7:2, 9).
Everyone who is able to give rational consent may marry (Heb 13:4; 1Ti 4:3). Yet Christians are to marry in the Lord (1Co 7:39). Therefore, those who profess the true religion should not marry unbelievers or idolaters. Nor should the godly be unequally yoked by marrying those who lead evil lives or hold to damnable heresy (Ne 13:25-27).
Marriage should not occur within the degrees of blood relationship or kinship that are forbidden in the Word (Lev 18). These incestuous marriages can never be made lawful, so that the individuals may live together as husband and wife, by any human law or consent of the parties involved (Mk 6:18; 1Co 5:1).