When did I miss this?

Why do couples move in together after only a month or two of dating? I know lots of women who have moved entire families in with a guy after only a few weekends. I know one woman who moved herself and her two kids in with a guy the day she moved them out of the father's house. Now, I have special objections to the whole living together with kids scenario, but in general when did living together become a casual thing?

I thought that moving in together was supposed to be a big step, a sign that things were getting really serious. Like, the next step is marriage sort of serious. But it seems to have become the norm now. If you're dating and it's exclusive, you live together. And if you have kids, well the theory seems to be that children need two parents so much that it's better to rush into finding a replacement than to wait and make sure the replacement will be a permanent one. So what you end up with may very well be a 12 year old who has already gone through four or more "dad's".

Tom and I lived together for about 2 weeks before we got got engaged, and I'm slightly ashamed of that fact. Why did I expose my daughter to a father figure without the commitment that would have (and eventually did) make him and actual step-father? Now, in my defense he probably only spent 3 or 4 days actually living here before we married, since he was driving over the road at the time, but I still don't know why I allowed it.

I don't bring this up because of any one couple but rather because I notice a trend with celebrities and in real life where couples don't wait for the shine of new romance to dull and instead they jump in with both feet, all while under the influence of first-kiss endorphins. But why? Why not wait and make it a big step?