Did God change? How is it that God that was so quiet about fornication, adultery, and polygamy in the Old Testament and then suddenly changed in the New Testament?
It is in the Old Testament that we learn that a man is to leave his family, get married, and become one flesh with his wife (Genesis 2:24). In illustrating the decay of mankind, we learn about Lamech, the first recorded polygamist who boasted about killing a young man (Genesis 2:19-24). It is Abram who is taken to task by Pharaoh for almost causing him to commit adultery (Genesis 12:17-20). It happened again with Abimelech (Genesis 20:1-9). Abimelech told Abraham, "You have done to me things that ought not to be done." It is also in Genesis that we learn that homosexuality was a grave enough sin to cause God to destroy four cities which were involved in it (Genesis 19).
Shechem, the prince of the city of Shechem, commits fornication with Dinah, the daughter of Jacob (Genesis 34:2). Her brothers declare "such a thing ought not to be done" (Genesis 34:7) and they ended up murdering all the males in the town of Shechem. Judah has sex with a prostitute after his wife dies and tries to hide the fact (Genesis 38). It turns out the prostitute was actually his widowed daughter-in-law and his sin became publically known. While he technically married Tamar, he refused to have sex with her again.
Joseph was solicited by Potiphar's wife to commit adultery with her. He refused because it would be a sin against God and his master (Genesis 39:9). In Genesis 35:22 we read about Reuben committing incest by laying with his father's concubine, which eventually led to his rejection in receiving the blessing (Genesis 49:3-4).
These are just the stories from one book, but it should be clear that sexual sins were never accepted by God.
When Moses brought Israel the law, it included a commandment against adultery (Exodus 20:14). Like the other Ten Commandments, it carried a death penalty in certain situations. Adultery was punishable by death of both the man and the woman (Deuteronomy 22:22). Fornication with an engaged woman was punishable by the death of both the man and woman if it was consensual or by only the death of the man if it was rape (Deuteronomy 22:23-27). Fornication by unmarried couples was punished by a fine of 50 shekels of silver (over three years of wages) and a possibility of forced marriage (if the woman's father consents) with no allowance for a divorce.
God chose to regulate polygamy by demanding that each wife must be treated equally. "If he takes another wife, he shall not diminish her food, her clothing, and her marriage rights. And if he does not do these three for her, then she shall go out free, without paying money" (Exodus 21:10-11). If you consider the law, you would realize that in most cases it would be impossible for a man to do as God demanded, especially if he was already in a healthy marriage. As a result, the first wife had a ready reason to leave the marriage if she so chose, leaving the man with only one wife. A polygamous marriage could only exist if the first wife agreed to it. God specifically forbade polygamy for kings. "Neither shall he multiply wives for himself, lest his heart turn away; nor shall he greatly multiply silver and gold for himself" (Deuteronomy 17:17).
The Old Testament documented mankind's sins, demonstrating repeatedly that man is unable to save himself. These violations do not mean God accepted the sins. As with many sins, God clearly states why various sexual sins are wrong and warns that He demands that mankind give up its sinful ways (Acts 17:30). This was not a change of His mind clarification of what He always expected of people.