As for Biblical references that support monogamy, here are a few that I think are pertinent:
In
Matthew 19, Jesus says:
4 “Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’ 5 and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? 6 So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.” (my bold).
Note that the passage says that a man is to be united to his
wife; not
wives and that the
two; not three, four or more shall become
one flesh.
In that same chapter in Matthew, Jesus also comments on Old Testament practices that seem to condone having multiple wives; here is the relevant passage in context:
3 Some Pharisees came to him to test him. They asked, “Is it lawful for a man to divorce his wife for any and every reason?”
4 “Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’ 5 and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? 6 So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”
7 “Why then,” they asked, “did Moses command that a man give his wife a certificate of divorce and send her away?”
8 Jesus replied, “Moses permitted you to divorce your wives because your hearts were hard. But it was not this way from the beginning. 9 I tell you that anyone who divorces his wife, except for sexual immorality, and marries another woman commits adultery.”
Now Verse 8 and 9 do not specifically state that polygamy is wrong- and a literalist might argue that a man can have another wife as long as he proclaims himself married to her, and doesn't divorce his first wife- but I don't think this is the intent of Jesus' message.
And if you think that any of this applies only to men, then
Mark 10 sets us straight:
10 When they were in the house again, the disciples asked Jesus about this. 11 He answered, “Anyone who divorces his wife and marries another woman commits adultery against her. 12 And if she divorces her husband and marries another man, she commits adultery.”
Another passage that supports monogamy is
1 Corinthians 7:
1 Now for the matters you wrote about: “It is good for a man not to have sexual relations with a woman.” 2 But since sexual immorality is occurring, each man should have sexual relations with his own wife, and each woman with her own husband. 3 The husband should fulfill his marital duty to his wife, and likewise the wife to her husband. (my bold).
Again, the emphasis is on a husband having one wife, not wives; and a wife having one husband, not husbands.
As has already been stated by others, another passage that promotes monogamy is the one in
1 Timothy 3 regarding the behaviour of overseers and deacons:
1 Here is a trustworthy saying: Whoever aspires to be an overseer desires a noble task. 2 Now the overseer is to be above reproach, faithful to his wife, temperate, self-controlled, respectable, hospitable, able to teach, 3 not given to drunkenness, not violent but gentle, not quarrelsome, not a lover of money. 4 He must manage his own family well and see that his children obey him, and he must do so in a manner worthy of full respect. 5 (If anyone does not know how to manage his own family, how can he take care of God’s church?) 6 He must not be a recent convert, or he may become conceited and fall under the same judgment as the devil. 7 He must also have a good reputation with outsiders, so that he will not fall into disgrace and into the devil’s trap.
8 In the same way, deacons are to be worthy of respect, sincere, not indulging in much wine, and not pursuing dishonest gain. 9 They must keep hold of the deep truths of the faith with a clear conscience. 10 They must first be tested; and then if there is nothing against them, let them serve as deacons.
11 In the same way, the women are to be worthy of respect, not malicious talkers but temperate and trustworthy in everything.
12 A deacon must be faithful to his wife and must manage his children and his household well. 13 Those who have served well gain an excellent standing and great assurance in their faith in Christ Jesus. (my bold).
So here Paul is providing advice for how overseers and deacons are to behave- but I would suggest that he is also promoting behaviour that all of us should aspire to live up to.
Another passage that indirectly speaks to this issue is
Ephesians 5:
33 However, each one of you also must love his wife as he loves himself, and the wife must respect her husband. (my bold)
I would ask, especially in the context of today's society where we recognize that men and women are to be treated with equal respect, how someone that respects and loves his spouse would entertain sharing that love with another partner- especially if the spouse is not in agreement with such an arrangement; I mean, doesn't the Golden Rule, as stated so beautifully in
Matthew 22 apply here:
37 Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’ 38 This is the first and greatest commandment. 39 And the second is like it: ‘Love your neighbor as yourself.’
How can you love your neighbour, including your wife, if you insist on sharing that love with someone else.
But for me, the strongest passage that supports monogamy is the 7th Commandment, given in
Exodus 20, where God states unequivocally that:
14 “You shall not commit adultery."
Where adultery is defined as sexual intercourse between a man and a married woman who is not his wife. Again, those who wish to interpret this passage for their own selfish desire will claim that having sexual relations with someone other than your first wife is o.k. as long as you proclaim yourself "married" to the second, third, etc. woman.
In all of this, I think a powerful additional source of advice comes from the Holy Spirit, who dwells in every Christian from the moment they accept Christ as their Saviour. For me, the Holy Spirit makes it crystal clear: a husband should have only one wife and love and respect her with all his heart and soul for all the days of his life. Same applies to the wife.
