But fornication is prohibited in the Bible. Gang rape is a forceful fornication. It is worse than fornication. So, it is already rebuked in the Bible. But, polygamy is not rebuked.
A woman who has been raped has not sinned. The crime of rape is that it is a forceful assault and humiliation that has devastating consequences. Not because it is a form of fornication.
There is only one chapter in the Torah which addresses rape as a crime.
Deuteronomy 22:25-27
"But if in the field the man finds the girl who is betrothed, and the man seizes her and lies with her, then
only the man who lay with her shall die. But you shall do nothing to the girl; the girl has committed no sin deserving of death, for just as a man rises against his neighbor and murders him, so is this case; for he found her in the field, and the betrothed girl cried out, but there was no one to save her."
Deuteronomy 22:28-29
"If a man finds a young woman, a virgin who is not betrothed, and he seizes her and lies with her, and they are found out, then the man who lay with her shall give to the young woman’s father fifty shekels of silver, and she shall be his wife because he has humbled her; he may not divorce her all his days."
This second passage is controversial for two reasons.
1. There are those who say that even though it uses similar words, the second passage is not referring to violent rape, but to seduction. The argument for this side is that the penalties are so different. In the first example, the penalty for the man is death. But in this second example, all he has to do is pay her father 50 shekels and has to provide for her without divorce. The counter argument is that the reason for the disparity in penalty is entirely due to whether the victim was married or not. If that's the case, then it would seem that the crime of rape is not a crime against the woman, but against the husband or the father. That sounds pretty ugly to me.
2. It is common for people to incorrectly interpret the passage to mean the woman is forced to marry her rapist. That is not the case. No woman is forced to marry her rapist. However, there were certain societal realities, among them being that a woman who was not a virgin was usually considered unmarriageable, and an unmarried woman was absolutely destitute. This law gives the woman the option of choosing a marriage that, while not what she wants, she might consider to be better than to try to survive alone.
Now let's address your comment about polygamy never have a law against it. There are two ways of seeing this that occur to me.
1. There are many instances in the Bible where deplorable things are mentioned, and never said to be wrong. For example, Sarah decides that her maid is going to have sex with Abraham, and Abraham goes along with it. The woman is a slave required to obey, so her opinion is of no importance; she therefore lacks the capacity to consent. Where is God's condemnation? It just never happens. We learn from this that silence on an issue does NOT imply that something is right.
2. As our culture evolved, a deeper understanding of ethics has evolved.
Among many other things, we understand today that the thoughts, feelings, and well being of women is something that MUST be respected. This was simply not the case back then. There are a long list of instances where women are simply told, have sex even if you don't want to, marry this guy even if you don't want to, etc. The men make the decisions. The woman's response is considered so insignificant that it is not even mentioned.
Polygamy is one of those things that is good for men, and not good for women.
I realize this is a generalization, so there are exceptions. I'm only relaying realities here, not passing judgment on anyone. Men have a much easier time with casual sex, and a great many of them seem wired to prefer having many different women, either at the same time, or serially. Woman have a hard time pulling sex apart from feelings and relationships. This is why we are so notorious for starting a sexual relationship meant to be casual, and then falling in love with the guy.
There is a joke I heard in San Francisco that illustrates these gender differences. When two gay guys get together, the first thing they do is exchange little black books. When two lesbians get together, the first thing they do is hire a moving van.
Returning to polygamy, a man may be highly satisfied by such a relationship, as it meets his instinct to spread his DNA to as many women as possible. But it absolutely destroys the woman's instinct to have an emotionally intimate relationship. To be frank, the more wives a husband has, the less able he is to fulfill the emotional needs of his wives.