Children who grow up with cohabiting couples tend to have more negative life outcomes compared to those growing up with married couples. Prominent reasons are that cohabiting couples have a much higher breakup rate than do married couples, a lower level of household income, and a higher level of child abuse and domestic violence.
Sounds good, right? I mean, married couples raise children with less negative life outcomes. So more marriage = better families and better lives for kids? Not necessarily. In fact, this is a perfect example of correlation not implying causation. Just because married partners do a better job of raising their kids than unmarried partners doesn't imply that getting married will make two people better parents. In fact, when you say it that way it sounds kind of crazy.
Take Alice and Bob (sorry, I know I shouldn't use these names unless I'm talking about the firewall paradox or encryption protocols), a couple of crack addicts who are madly in love with each other as long as there is enough money to stay high, but whose chemically-enhanced bliss devolves into a melee after a day without a visit from their dealer. Lately they have had to choose between buying condoms and buying more crack, and are soon to discover the consequences of their decision. How good a parenting pair will they be? What if they got married?
Take Carl and Doug (I don't know the standard names past Alice and Bob!), Carl works for a software company and Doug is a moderately successful artist (the kind that wouldn't starve even without Carl, but that isn't famous enough that anyone would recognize the name Doug). They love spending time with their nieces and nephews, whom they babysit regularly. They are thinking of adopting kids and have started combing the parenting section of Amazon books. Would they be better parents if they lived in California (and hence had gotten married) or if they lived in Texas (and hence were "just roommates")? For that matter, what if they lived in California but just didn't believe in marriage? Is that a strike against them?
Maybe the kinds of people who are interested and invested in raising their kids the right way (and would therefore be predisposed to be good parents) are also more likely to be enchanted by the romantic ideal of being married and raising a family a la the Cleavers. Maybe. Maybe not. But I doubt the actual act of getting married has much to do with how well adjusted a couple's children will be, whether it's legal or not in your state.
Think of this the next time you hear a correlation used as evidence of causation in a political debate or column. It'll save you time by letting you wave your hand dismissively at the monitor/TV screen and mutter "Cum hoc ergo propter hoc", because correlation does not imply causation!