Why is rape not in the 10 commandments?

Because the 10 Commandments have little to do with ethics and everything to do with (a) creating an orderly society and (b) distinguising the Jewish community from its neightbours. Women were generally regarded as property by the culture of the time, which wrote the commandments.

I've heard that the 10 commandment is there for "category" of prohibition. Rape would go to don't steal I guess. By raping you are stealing "usage right" from the girl or the parent.

In a sense rape is indeed prohibited. In ancient hebrew, rape is actually punished by fine that can be quite high. That's how we define something as prohibited, namely that there is a punishment.

The difference is on what's the punishment and how do we measure damage.

In ancient time they don't have jail. Jail is a very expensive innovation. So the only punishment besides stoning possible are either fine or canning.

Jewish halakha actually prescribe that the rapists must pay for the pain and for the drop of the market value of the girl. The 50 shekel is only for the pleasure

http://www.chabad.org/library/article_cdo/aid/960635/jewish/Chapter-Two.htm#footnote2a960635

Damages [are evaluated] according to [the girl's] beauty. We look at her as if she were a maid-servant being sold in the marketplace:

The judge would estimate how much the girls' market value is. Maybe the judges would auction of the girl and shill bid it to get the correct market value of the girl as sex slave. Maybe they just estimate. Got to ask the jews again.

So raping Selena Gomez is still off the chart. In ancient time people do not have bankruptcy laws. So if you can't pay you become slaves, which is like going to jail.

https://judaism.stackexchange.com/questions/11251/why-does-the-rambam-maimonides-equate-all-sex-outside-marriage-as-prostitution/11276#11276

Some may think it's disgusting. However, we may have to consider:

1. Inevitability of power. Even nowadays, women are not absolutely free to decide what they do with their body. Prostitution is illegal for example. So is group marriages. Hence we shouldn't judge a culture where parents control their daughter as disgusting given that in our modern culture the states have a right to control all women, most of which are not daughters of the president.
2. Ancient jewish culture is more hierarchical. Western civilization follow the greek tradition that humans are equal. That's why raping an ugly girl is just as evil as raping a beautiful girl now. Ancient culture tend to actually be more objective on this. Raping a beautiful girl of course cause more harm than raping an ugly one just like destroying a Ferrari causes more harm than destroying a bike. Also stealing a Ferrari is more tempting than stealing a bike and hence need to be punished more. Contemporary court have this beauty blind attitude as if beauty shouldn't matter at all.
3. Women at that time usually live at home. Think of muslim society. Ancient jews are like muslim. Not easy to rape a girl.
4. Probability of getting pregnant is quite low if it's just from one intercourse. Losing virginity would drop the women's market value and that's the real damage of rape + pain damage.
5. In polygamous society, which ancient judaism is, there is always a shortage of women. In fact, anthropologists and evolutionary psychologists claim that it's the real reason why we have laws against polygamy. So yea, the girl can still aim high because there will still be plenty of rich males for her, each want unlimited number of young girls. http://www.psychologytoday.com/articles/200706/ten-politically-incorrect-truths-about-human-nature

6. Rabam seems to think that harlots cover all sex outside marriage. However, the issue is fatherhood determination.

   "For [ultimately], a father will marry his daughter and a brother his sister, [for in a sexually permissive society] a [girl] may become pregnant and give birth without knowing who the child's father is."

   • I forget the URL. It's really there. So perhaps the commandment to get married is there to ensure that should the girl get pregnant somebody will support the child. However, the parent could and most likely would still reject the boy if the boy is poor anyway. If the boy is rich, then he wouldn't need to rape right? Not like there is a strong intensive to rape either case.

I will have to ask the jews again now that we can decide fatherhood by DNA tests, what would Rabam says about sex outside marriage and rape.

Jews do not think sex outside marriage as a big deal. It never get death penalty. Only on some circumstances it can get pretty serious.

You can read here: https://judaism.stackexchange.com/questions/11021/what-does-adultery-mean-in-the-7th-commandment/11032#11032

Which explains why prostitution is legal in Israel I guess.

It is:

No. 7 Thou shalt no commit adultery

Rape is a form of adultery (unless you're married).

Jesus tells us how to interpret the commandment not to kill, namely that it is even a sin to call your brother stupid. I think it is fair to say that Jesus generalized that commandment to one that absolutely forbids violence against persons, be it physical or "only" verbal.

Of course, this would also cover rape.

Not every sinful act is spelled out in the ten commandments, but every possible sinful act does fall under one or more of the umbrellas. Rape is adultery as well as theft and envy and does not honor the Lord. That's at least four "counts". How much does one need to know that it's wrong?

In the NT we find the ten commandments expounded to include thoughts about the action and heart attitudes as well as the spelled out actions. In short, not having the act of rape spelled out in a commandment is not an oversight, the topic is well covered. I have never heard it alleged that Christianity allowed rape based on it not being the 11th commandment.