Sources of knowledge about Indian custom include sastras in addition to Buddhist texts. I am going to give some rough answers based on the Arthasastra (3rd century BCE).
**Obviously I did not invent this social system…and neither did the Buddha. There are also class and social status factors involved too.
As the 3rd precept primarily prohibits behaviour that is also illegal in ancient law systems, the Arthasastra is a useful cross reference. Sexual behaviours tend to be illegal in ancient legal systems when a 3rd party right is violated or when social stability is threatened. As this is patriarchal system, “rights” generally means, “patriarchal rights” and social stability means, “patriarchal social stability”. The Arthasastra very clearly gives women some rights (the protection of women obviously being one of the reasons that law exists…), but these are circumscribed within the family system.
Polygyny is mostly an upper class practice historically, not a practice of the masses. EDITEDAbout 3.2% of Indian Buddhists have polygynous marriages in 2006 (slightly higher rate than Hindus…down from 7.6% in the 1960s) despite it being outlawed in the 1950s in India under a legal code that lumps Buddhists together with Jains and Hindus. The reason it could be outlawed in India was that polygyny was held to NOT be an essential part of these religions. Poverty may factor into the current polygyny rates today, as being a 2nd wife can provide social security for women who otherwise may find it hard to marry.
Additionally, the ancient legalists (sastra authors) have tended to have a more restrictive view of polygyny than the practice of the upper classes in general (polygyny with reason only).
E.g. the Arthasasatra gives the reason for taking a 2nd wife as being failure to produce a son after a set period of time. Bearing in mind that marriage in ancient India was explicitly for sons…this was the drive for marriage in the first place.
I have never seen anything about the first wife’s consent anywhere. The need for a son is considered so highly in many ancient legal systems that it is given as a reason to disobey parents. Ancient Indian legal systems also allow for fines for parents who prevent their children from having children.
So my instinctual feeling is that the 1st wife has no right to object in this case as need for son trumps even obligation to parents.
Extramarital sex (for men) is looked down on in Indian culture. It is not normalised. We can see this in Buddhist texts that discourage womanising. But I don’t know of any text where it is illegal en toto, because there is no violation of a 3rd party right (the wife not having a legally recognised independent right). 
Women could also be punished for bad behaviour, including consensual extramarital/premarital relations or disobedience.
South Asia has historically operated on the “men are like gems, women are like cloth” principle (this is a saying from Cambodia). Both are precious, but men clean up more easily if there is a stain. So social regulation of women was/is stricter. This was also before effective contraception, and in an era when the state had less policing ability in general. There was a generally recognised pressing need to prevent social chaos from unwanted babies, and the belief that best way to do this was the social control of women.
As noted by Cabezon, there are several attitudes to sexuality in Buddhism today. One is that the original values and practices of the texts still hold good unchanged. This view is widely held in Asia, but due to colonialism and other pressures, polygyny is less common.
(Although some Asian Buddhist cultures are matriarchal and clearly have local practices going on, including polyandry.)
The second is that the values of the texts are still good, but may need some modifications or adaptions.
The third view is that the whole system should be reworked, retaining only its most minimal features, like consent and spousal fidelity (following Western legal and liberal social thinking in general.)
Views no. 2 and 3 are common in English speaking circles. There is no serious Buddhist pro-polygyny lobby in the West.
