There are those who argue that since Islam permits Muslim men to have sexual intercourse with their slave girls, this then means that they also have the right to rape them.
This is absurd. The right to have sex with a woman does not necessarily imply that one has the right to rape her as well. To say that a Muslim man has the right to rape his slave girl is like saying that a man has the right to rape his wife; which is not true.
Rape in Islam is completely forbidden.
For a detailed discussion see article here