Marriage should be a mutual agreement that should not harm the well-being of both parties. For marriages that fall outside of the norm, such as same-sex marriages and polygamy, if the parties involved in it are happy, i feel that no one would have the right to prevent it.
An ethical concern would be how the brides are mistreated. For instance, for dowry deaths, the bride is mistreated when her family is unable to pay enough dowry to satisfy the groom's family. A human life is unnecessarily sacrificed over material gains. Also, for child marriages, most children are involved in human trafficking. This eventually results in domestic violence and abuse.