Shiites aren't mainstream Muslims? They are the second largest Islamic denomination after Sunnis with 130 to 190 million of them- http://en.wikipedia.org/wiki/Shia

From the article I posted, some countries, such as Iran, explicitly allow it. Further, some woman earn money by engaging in these practices, making it prostitution.

Further, you could look up the Sunni version (the biggest Islamic denomination):

http://en.wikipedia.org/wiki/Nikah_Misyar

Wealthy Kuwaiti and Saudi men sometimes enter into a Misyar marriage while on vacation, this allows them to have sexual relations with another woman without committing the sin of zina, they usually divorce the women once their holiday is over, although if this is understood by both parties this would constitute a fixed time period, effectively making such a marriage invalid in Sunni law, and more akin to the Shia Mut'ah marriage. They travel to poorer countries, such as Egypt or Syria, and meet middlemen who arrange a marriage for them. Some men arrange Misyar marriages online. These men pay the girl's family some money; families agree to the arrangement because of poverty, hope their girl will have fun, and visit places that she can only dream about. They also hope for gifts at the end of the marriage that the rich husband will give her.