Assalamo alikom
I'm so sorry, but who told you that??This is not true.
If you are talking about the non Muslim women who were taken as captives during Islamic wars, this gonna be different issue.
First of all, taking women as captives in wars and having the right to deal with them as wives was a very past habit before Islam. Islam spares no effort to get rid of slavery by making the expiation of many sins is to release a neck( slave).
Allah,swt. enjoined Muslim men to deal very kindly with those captive women who may have lost their family in war. Yes, the Muslim men had the right to deal with them as if they were their wives because they became in their houses. If this woman got pregnant,the Muslim man should then release her and give his child all his/her rights.
All this is over now and you can not have a slave or a servant to do such things with her. This is completely forbidden.
If I said something wrong, I hope that my brothers and sisters correct me.