Baby or pregnancy THEN marriage? Why follow up a mistake with another?

I'm astonished with just how many 20 year olds on facebook right now post baby pictures before getting married. And the thing is, many of these unplanned pregnancies happen very early on in the dating game, so why are people compensating for their mistakes by getting married? I dont mean to sound cynical, I'm sure SOME of these people are in love, etc, but its hard to believe that this many people are truely getting married for the 'right' reasons.