Zeldafan is pretty on the nose, the Old Testament is really fond of people with tons of kids. That's where the Quiverfull movement comes from, the verse that says that men with lots of kids are like a mighty warrior with a ton of arrows, and the more kids (arrows) he has the cooler he is. And since with only one wife you can only have one kid at a time, the solution to this is pretty clearly multiple wives for more kids!
There's another side to it though, which is pretty well known via the story of Onan. Women weren't considered equal to men, they were second-class citizens and as such, if a woman's husband died before she had a son by him, she was kind of out of luck as far as someone to take care of her. Thus, the solution to this was that if your brother died before his wife gave him a son, it was your duty to take her as your own wife. Since this wouldn't really work in a monogamous society, you'd have to allow for polygamous relationships. If a man refused to marry his brother's widow, there was a whole ceremony they'd have to go through where she'd spit in his face and rip his sandal off his foot and say "This is what is done to the man who will not build up his brother's family line" and his family would be known as the Family of the Unsandaled.
Rusty Claymore wrote:While the Bible doesn't literally, specifically, IN ALL CAPS, say, "DON'T MARRY MORE THAN ONE! SRSLY!" Polygamous stories don't end well.
Actually Deuteronomy specifically forbids kings from having multiple wives. This would prove itself to be an excellent warning in the case of Solomon, who had hundreds of wives and who are blamed for leading his heart from God and introducing foreign religion to Israel. But other than that, polygamous stories don't necessarily end poorly. Esau had two wives and everything went fine for him, and Moses is said to have had 3 wives and he turned out alright.