Was fornication not a sin under the Old Testament?

Answer:

There were numerous types of offerings given to God depending on the circumstances. A sin-offering was given when the person's sin did not have restitution included in the punishment.

"Now if a person sins after he hears a public adjuration to testify when he is a witness, whether he has seen or otherwise known, if he does not tell it, then he will bear his guilt. Or if a person touches any unclean thing, whether a carcass of an unclean beast or the carcass of unclean cattle or a carcass of unclean swarming things, though it is hidden from him and he is unclean, then he will be guilty. Or if he touches human uncleanness, of whatever sort his uncleanness may be with which he becomes unclean, and it is hidden from him, and then he comes to know it, he will be guilty. Or if a person swears thoughtlessly with his lips to do evil or to do good, in whatever matter a man may speak thoughtlessly with an oath, and it is hidden from him, and then he comes to know it, he will be guilty in one of these. So it shall be when he becomes guilty in one of these, that he shall confess that in which he has sinned. He shall also bring his guilt offering to the LORD for his sin which he has committed, a female from the flock, a lamb or a goat as a sin offering. So the priest shall make atonement on his behalf for his sin" (Leviticus 5:1-6).

The list of sins in this verse served as examples. It was not intended to be a complete list of all possible sins. The examples all show sins that were not premeditated and purposely carried out. These were crimes of the moment, either from lack of thought, accidental, or weakness of self-control.

A similar sacrifice was called the guilt-offering. This one was given for an unintentional sin where the sinner was also required to make restitution for his sin. Some examples given were:

• Causing damage to a holy thing (Leviticus 5:15-16)
• Lying (Leviticus 6:2-7)
• A leper who has been cleansed (Leviticus 14:10-28)
• A sexual sin with another person’s slave (Leviticus 19:20-22)
• Breaking a Nazarite vow (Numbers 6:12)

Once again, the list is not intended to be an exhaustive list of all possible sins.

Fornication between unmarried couples carried a penalty requiring a dowry of 50 shekels of silver. A silver shekel was worth what an unskilled laborer could earn in 40 days of work. The required dowry price would be roughly what an unskilled laborer could earn in seven years (assuming he had to live on some of his earnings). In addition, he was required to marry the woman with whom he had sex with no allowance for a future divorce unless her father objected to the marriage (Deuteronomy 22:28-29; Exodus 22:16-17). This sin would also require a guilt-offering to be made.

Therefore, the premise of your question is false and the conclusion is also false.