I'm not sure which of the three is more distasteful. Is selling a kidney or other organ for money more disgusting (or sad) than contracting a sham marriage for financial gain? Is switching one's faith for money sadder than receiving money to marry someone whose only aim is to migrate to the UK or the USA?
Personally speaking, I would put organ donations for money on top of this list. It must be really painful to be in a position where one is forced to donate an organ, maybe a kidney, to earn some money. Mind you, in India the Transplantation of Human Organs Act, 1994 makes it illegal to donate an organ for money. This law is observed more in breach. Desperate men and women continue to donate their organs for money. As long as there are willing buyers and eager buyers, this trade will go on. Rather than try and prevent it, it would make sense for the authorities to merely make sure donors are not cheated when they donate an organ.
People get married for reasons that vary from love to a need to please one's parents to getting money or property. Some get married because it is the done thing. In most cases, it is a combination of some of these reasons. I assume more than a few marriages are entered into solely for monetary reasons. Therefore, it should be no surprise to know that there are many people, nationals of countries like the UK and USA which are high up on the migrants wish list, willing to enter into contracts of marriage with wannabe migrants solely for money. Detecting sham marriages is taken seriously in these countries. Last year, the UK even tried to make a law which required foreigners living in the UK (other than permanent residents) to seek special permission to marry, irrespective of the status of their partner. The House of Lords struck down this law, but migration authorities continue to have the right to delve into a marriage to see if it is a 'sham.'
In my opinion, as long as a person is legally entitled to marry, no one should have the right to question his or her reasons for getting married. Why is it acceptable to get married for a fat dowry in India or a farmhouse in Surrey, and unacceptable to marry with a view to facilitating a migrant's entry into one's home country? As long as both parties are legally entitled to marry, that is, they are of sound mind and not already married, it should be none of anyone's business.
People change their religion for various reasons. In my opinion, the most common reason in modern times is – love. I've known so many instances of people switching faith in order to get married. This usually happens when one party in a marriage comes from a very religious background and his or her family will be terribly unhappy if he or she marries someone from a different religion or sect. And so, the other party, when faced with the prospect of losing the love of one's life, agrees to switch faith. Despite so much controversy in India over conversions, no one has seriously challenged the right to convert for love.
Then there are conversions for social advancement. By social advancement, I don't just mean escaping the caste system. Unlike the British, the Portuguese and the Spanish considered conversion to be one of the goals of colonisation. Therefore, during the Portuguese rule in Goa, many conversions were achieved by force. But not all conversions were forced. There were many who wanted to be be on the side of the ruling class, with the expectation of various benefits. Something very similar must have happened during Arab/Mughal rule in India. Some of those who converted may have been forced. Many others must have converted for social advancement and other benefits.
In recent times, practically every religious community in India has indulged in conversions. Various Hindu sants have converted tribals and dalits to mainstream Hinduism and reconverted those Hindus who had converted to Christianity. Christian priests have continued their noble task of spreading the Word and Muslim preachers have not been far behind. Interestingly, the growing economic clout of the overseas Indian/Hindu community has meant that Hindu missionary activities outside India have gained momentum. One only has to walk past Oxford Circus in London to see a number of 'white' Hare Krishna devotees singing and dancing and preaching. The Hare Krishnas and various Hindu temples in the UK such as the Sri Mahalakshmi temple in East London, serve free food to the hungry and run schools. I believe this practice is followed in the USA, Canada, Australia etc. In the West, no one questions the right of the Hare Krishnas or other Hindu organisations to conduct these activities which are not much different from the activities of Christian missionary and Muslim organisations in India. Like Christian missionaries in India, Hare Krishnas face flak in some intolerant parts of the world such as Kazakhstan. When a local authority in Kazakhstan destroyed part of the Hare Krishna settlement outside Almaty, there was a global outcry, including from the West.
Usually the ones who are willing to convert for benefits are not particularly religious. Having converted to a new faith, they do display the outward signs of that religion and bring up their children in the new faith. Usually the new faith sticks, though it may take a generation or two to do so.
I don't think religious conversions can be or should be banned. The Indian Constitution gives every Indian the freedom to practice any faith of his or her choice. This obviously includes the freedom to convert to any religion for any reason whatsoever. It could be for the purpose of getting married to someone, it may be to gain nirvana or salvation, it may be to get a job or to escape the caste system. But it should be none of anybody else's business.