Religion is so influential that even atheists talk about marriage as though it was normal. I've added some more details to the main page of AtheistInsurgency.com and in the process began to think about why government has such a strong hand in marriage.
Marriage is a religious institution, is not congruent with nature, allows the government to apply privileges and penalties to married couples that unmarried individuals would not experience, forces women into the beds of men they don't like, allows men to maintain authority and leverage over women, and reinforces a false sense of inferiority and sublimation in women.
Think about it. Just like there are many degrees of attraction between homosexuality, bi-sexuality, and heterosexuality, there are degrees of behavior between polygamy and monogamy. One could make an X-Y axis chart with the two sets of the aforementioned data sources and find the majority of intersecting points in the range of polygamous heterosexuals (if they only practiced unprotected sex). That would only make it cultural norm purely because of successful procreation.
The survival of our species depends on the willingness to raise the children we produce together. There was an advantage in numbers and large families once had a better chance at surviving when they were completely on their own, hunting and gathering.
Now there are social programs that appear to accommodate unprotected polygamy because religious institutions prefer abstinence only education, denial of access to contraceptives, and insurance coverage of Viagra but not birth control.
If we apply the logic of scientific findings in nature between procreating species, and extrapolate it toward human behavior, the value of marriage becomes obvious.
If we as atheists were to add to our platform the idea that marriage serves no real beneficial purpose, we would definitely be a threat to a multi-billion dollar business.