Quite often it's claimed that according to the Bible marriage is only meant to be between one man and one woman. Why? Because it forestalls fornication:
7 Now concerning the matters about which you wrote: “It is good for a man not to have sexual relations with a woman.” [Anyone here on D C and R care to venture a guess as to why?] But because of the temptation to sexual immorality, each man should have his own wife and each woman her own husband.
So we have remedial verses such as:
24 Therefore a man shall leave his father and his mother and shall become united and cleave to his wife, and they shall become one flesh.
1 Now concerning the things whereof ye wrote unto me: It is good for a man not to touch a woman.
2 Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
However, from what I can determine, the word "only" never enters into the instruction, just a "shall" and "let." And, as it turns out, as used in the USA “shall” is not a word of command or obligation. In addressing its meaning, in Gutierrez de Martinez v. Lamagno 515 U.S. 417 (1995). the Supreme Court of the United States ruled that “shall” really means “may.” More importantly, if "shall" was an imperative, a command, it wouldn't leave room for bachelors and spinsters, neither of which are denounced in the Bible or, to my knowledge, by conservative Christians----just consider Jesus who was accepted as a 30 year-old bachelor.
So marriage only between a man and a woman is not commanded, just strongly suggested, which is no doubt why the Bible is replete with accepted exceptions, and provides further evidence that "shall," as used in the Bible verse above, is not a command or expresses any obligation.
The exceptions: (source: video below)
"There's polygamous marriages, like Lamech who married two women, Adah and Zillah. And Ashur who had 2 wives, and Esau who had 3 . In fact, Gideon, a leader of the Israelites had enough wives to bear him 70 sons., and David, of David and Goliath, had 8 wives. So there are many Biblical marriages that go beyond one man and one woman. Sometimes by large orders of magnitude. But the Bible doesn't go out of its way to say those people are doing anything wrong. There's no judgement or condemnation. Sometimes god is even proud of it. In fact, Jacob had two wives. Now god could have punished him, but no." So going against his suggestion in Genesis 2:24 is no biggie.
More on this issue in the following video.