While in an archaeology museum, I recently came across a grave marker for a woman who married her former master and had two sons with him. I was under the impression that the master could take the slave as a lover but wouldn't marry her, this marker seems to contradict that. Any information would be immensely helpful.
In ancient Rome, slaves could not legally marry, not even among each other. So to marry his slave, a Roman would first have to free her. This was actually one of the few exceptions in the Lex Aelia Sentia of 4 CE, which put limitations on all other kinds of manumissions. If the citizen wanted to do so, he would have to appear before a tribunal and state his case. The slave would then be declared free and the two could marry.
It was not unusual for male owners freeing and marrying female slaves. However, female mistresses freeing and marrying male slaves was frowned upon and in the later empire made illegal. Another prohibition existed for members of the senatorial aristocracy, who could not marry freed slaves either. They usually took them as concibines.
Source: Grubbs, Judith: Women and the Law in the Roman Empire: A Sourcebook on Marriage, Divorce and Widowhood. London and New York, Psychology Press, 2002 - ISBN 0415152402