Civil marriage (which is what we are discussing), is a social contract between two people that the government endorses for its own benefits. Some of these benefits include, but are not limited to, improving the odds that children will be raised by at least two people, most likely their mom and dad, helps to provide stability, gives the government someone to essentially "bill" for any debts/social obligations that a person has/leaves behind when they die (especially true when there are no blood relatives for the person), and it keeps track of who is actually married, to help to protect one party from getting cheated. There most likely are more.
From the way that the government treats heterosexual marriages, it is obvious that it is not necessary that all or even most need to apply to all marriages. Some of these same benefits could be gained from having homosexual marriages. That is hard to argue. They should not have to prove "extra" benefits that would come from their marriages, but some are even having to do that. For example, one of the questions the Prop. 8 judge gave to the pro-gay marriage lawyers is to "Is there proof that giving homosexuals the right to marry will reduce discrimination against them?".
Not evey heterosexual couple is even able by law to have children, so the argument that heterosexual marriage's main purpose is procreation is completely down the toilet. There are at least a couple of states where first cousins can marry only if at least one of them is medically or naturally sterile.