Statistically, women are more likely to be  phsycially abused if they are cohabiting rather than married.  The  likelihoode of it leading to marraige is very small, and among  cohabiters that DO end up tying the knot, they are FAR more likely to  divorce later on. The scientists who have run these studies have  intepreted the data to suggest that those who choose to cohabit have two  problems: 
1. They mistakenly assume that cohabiting will reveal if a relationship  will work as a marraige, when in reality people act very differently in a  committed marraige than a non-commital living arrangement.
2. Those who choose to cohabit do so primarily out of difficulties with  commitment, making them statistically more likely to divorce even if  they do eventually marry.
 Basically, if a stable marriage is your goal, the last thing you should  do is cohabit.  If cohabitation is what you really like, then it's  probably a good idea not to marry anyhow, as you are not likely to stick  around.
I could go into the issues of how this impacts children, but in my  experience, people who choose to cohabit don't usually care how they  effect their kids.
Yeap, I know my opinion is a strong one, and very counter-cultural.  But  you knew when you asked the question that there would be these  differences of opinion, eh?
Journal artical connecting cohabitatin with domestic violence: 
http://www.northwestern.edu/ipr/jcpr/workingpapers/wpfiles/kenney_mclanahan.pdf
Journal artical linking cohabitation with higher divorce rate: 
http://sf.oxfordjournals.org/content/69/1/207.short