Answer:
- Monogamy
In this form of marriage, two individuals committed themselves to each other sexually and emotionally. The partners in a monogamy are prohibited to have sexual relationship with one another and they can receive some legal benefit from the government. Currently, this is the most popular form of marriage.
- Polygamy
In this form of marriage, more than two individuals committed themselves to each other sexually and emotionally. The partners in polygamy can have sexual relationships with any person within the marriage, but not someone outside the marriage (unless there's explicit permission). Currently, this form of marriage still not acknowledged by US government.
- Open Marriage
In this form of marriage, two individuals committed themselves to each other emotionally, but each partners are allowed to have sexual relationships with other person.