Combined marriage is a form of polyandry that existed in the pre-Islamic period in the Arabian peninsula.
One ... type of marriage was that a group of less than ten men would assemble and enter upon a woman, and all of them would have sexual relations with her.
If she became pregnant and delivered a child and some days had passed after her delivery, she would send for all of them and none of them could refuse to come, and when they all gathered before her she would say to them "You (all) know what you have done and now I have given birth to a child.
[1]This form of marriage was outlawed by Islam, which requires that any man and woman be married prior to sexual intercourse.
In addition, Islam requires that the identity of the father be known, in turn prohibiting a woman from having sexual intercourse with more than one man, her husband.