Is there something about *Thou shalt not commit adultery* that you missed?
And while rape per se, is not mentioned in the Ten Commandments, is not a reflection of the importance of the issue. Lots of things are not mentioned in the Ten that are dealt with in other areas of the Law.
The Ten Commandments is not meant to be a throughout treatment of all people's relations with each other, as much as a summary.
It's all expounded on in the rest of the Law.
Old Testament culture held that children were the property of their father, so a girl could be given, bought, or sold to another man as his "wife" regardless of her age or her feelings on the matter. After which he could do as he pleased with her. When Muslims do this, we call it rape. When we read about it in the Old Testament, however, we seem to get all misty-eyed with nostalgia.
An Old Testament man with multiple wives (whom he could have sex with whether they were willing or not) probably hadn't the energy, but if he did, raping his slaves was quite permissible. And raping a virgin is "righted" by him marrying her.
Adultery is defiling another man's property, that's why it's forbidden. But the women who were your own property, of whom there could be as many as you could afford to obtain, could be raped at will.