I wouldn't consider morals about how we are supposed to act "knowledge", personally, would you? We're talking about the list of rules that God said was necessary to run a just society in the past. Some of them were specifically about worshiping and honoring God, but plenty of them were mundane laws about justice on Earth. Is there any reason you can think of that God would hide this moral from His people so that sexual abuse went undeterred? Otherwise this answer amounts to not much more than "He works in mysterious ways".
The short answer is that God did not "hide this moral from his people". This will explain:
The Bible does address the issue of rape. As expected, when the Bible mentions the crime of rape, it is depicted as a gross violation of God’s design for the treatment of the human body (Genesis 34). The Bible condemns rape whenever it is mentioned. For example, there is a particular passage in the laws given to the nation of Israel before entering the Promised Land under Joshua’s leadership. This passage (Deuteronomy 22:13-29) spoke directly against forcing a woman into a sexual encounter against her will, or what we know today as rape. This command was meant to protect women and to protect the nation of Israel from committing sinful actions.
Deuteronomy 22:25-27 mentioned the punishment the Mosaic Law commanded for a man who raped a woman. The man was to be killed by stoning while the woman was considered innocent. Though the Mosaic Law was for the nation of Israel during the time of Moses, the principle is clear that rape was sinful in the eyes of God and led to the most extreme punishment possible—death for the rapist.
There are some difficult passages in the Old Testament, however, in relation to this issue. Critics of the Bible are quick to point to Numbers 31 (and other similar passages) in which the Israelites were allowed to take female captives from nations they conquered. Critics make the accusation that this is an example of the Bible condoning, or even promoting, rape. However, the passage says nothing about raping the captive women. It is wrong to assume that the captive women were to be raped. Again, Deuteronomy 22:25-27 condemns rape, even advocating the death penalty for perpetrators of rape. In the Numbers 31 passage the soldiers were commanded to purify themselves and their captives (verse 19). Rape would have violated this command (see Leviticus 15:16-18). The women who were taken captive are never referred to as sexual objects. Did the captive women likely eventually marry amongst the Israelites? Yes. Is there any indication that rape or sex slavery was forced upon the women? Absolutely not.
In the New Testament, rape is not mentioned directly, but within the Jewish culture of its writers, rape would have been considered as sexual immorality. As such, both Jesus and His followers (including the apostle Paul) spoke against sexual immorality, even offering it as justifiable grounds for divorce when a person actively committed sexual acts outside of the bond of marriage (Matthew 5:32). This would not, however, apply to the victim of rape, only the one who committed the act.
Further, the New Testament is clear that Christians are to obey the laws of their governing authorities (Romans 13). Not only is rape morally wrong; it is also wrong according to the laws of our governing authorities. As such, anyone who would commit this crime should expect dire consequences, including arrest and imprisonment.
To the victims of rape, much care and compassion must be offered. God’s Word often speaks about helping those in need and in vulnerable situations. Christians would do well to practice these teachings and model the love and compassion of Christ by assisting victims of rape in any way possible.
To those who have committed rape, we must remember two things. First, people are responsible for the sins they commit. Second, however, no one is beyond the grace of God. Even in the lives of those who have committed the vilest of sins, God can extend forgiveness to any who will repent and turn from their evil ways (1 John 1:9). This does not remove the need for punishment according to the law, but it can offer hope, even to those whose sins have made them outcasts in the eyes of others.
http://www.gotquestions.org/Bible-rape.html
There is a phrase that means "to lay with against their will" not a word. Otherwise they would have a word for rape. That logic is a bit backwards to me. They invented a word for adultery which could have been surmised in the phrase "to lay with another man's wife" (since that's how they defined it back then). So, and true, this is speculation, it seems like they would invent words for things that they find important. But to think that they wouldn't make a law for something because they don't have a word for it? Why?
But, as you say, they did have a phrase that they used to describe it, but they only outlawed it in specific instances against specific types of women. They understood that sometimes women were forced against their will, but they only outlawed it some of the time. The only time that it is mentioned in the Law is when there is a man who is financially harmed by the woman being forced. Does this seem interesting to you?
Once again, scripture is absolutely clear about the consequence of rape. You should read about Amnon and Tamar in 2 Samuel 13. I would also read Judges 19 and 20 where the Levite servant was raped, as a result war was declared against the Benjaminites because they refused to turn over the men who raped the servant. If you read further, the Benjaminites were defeated and almost destroyed. I would challenge you to look at any example where someone raped another for any reason and read on to see the consequence of that rape. I guarantee that it is not pretty for the culprit. Just about everyone who raped another was eventually put to death....period.
