Although you can find regular families in India, what really abound are really large ones. And when I say regular families, I mean families with a father, a mother and their offspring. Nothing out of the ordinary as those 'modern' families (which some old-fashioned people don't even call families) of gays or lesbians with adopted children, as we have here in Europe.
In Indian culture the family is really important. Indians horrify at the western 'lack of family loyalty'. For them, a family is not only people of the same clan living under the same roof, but relatives joined together by rights and commitments and subject to a hierarchical authority among its members.
So it's not weird to ask someone how many brothers or sisters they have and to get '40' for an answer! That is called an extended or joint family. It consists of all persons lineally descended from a common ancestor, and includes their wives and unmarried daughters. A daughter ceases to be a member of her father's family on marriage, and becomes a component of her husband's.
So that you get the picture: a couple lives with all their sons and daughters. When the sons get married, their wives come to stay in the same house and there their children are brought up, too. And the daughters live there until they get married. At that momento they move to their husband's house.
The patriarch (usually the oldest man) makes decisions on financial and social matters. His wife exerts control over the kitchen, child rearing and religious practices. Old women have more competence than young men, though. All money goes to the common pool and all property is held collectively.
This has its advantages. They develop a feeling of strength and security. They know they'll never be alone; they'll always have their family's support. To get that support they just have to comply the rules and not to dishonour the family.
So imagine you got married to an Indian man. You'd move to his house with all your in-laws and without a saying in your relantionship! So, there you are obeying your parents-in-law blindly, and with no right to complain. If one day you fancy eating this or that thing, you can't! If you feel like shouting at your husband, you can't! If you consider your kiddo needs some punishment, you can't! If you'd like to save money to indulge in a whim, you can't!... As if a 'normal' marriage wasn't complicated enough!
If you that you add that polygamy is an extended practice in India,... Picture yourself in your in-laws' house with your husband's other wife/wives! A loony house!
P.S.: Polyandry is also in practice.