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