I've known a few people who have done this. It is my opinion that these people are destined for unhappiness, disaster, or both. First, lets start by examining the immediate effects. These people are going to need to establish well paying jobs. Going to college while married (not to mention if they have children) would be very difficult financially and emotionally. Very few people can succeed at this, so they fall back into low paying jobs that are available to all kids fresh out of high school. Unless, the parents are covering these kids, they are going to be trying to make rent payments on minimum wage (again, throw kids into the mix and its even worse). But lets assume for a moment know that the financial burdens are inconsequential. First off, in today's age people are not socially mature until they are into their mid 20s, at least. People marrying at the age of 18, are far from knowing what they want in life. In a few of the cases, the people getting married didn't even know what they wanted to do as careers. That is very troubling. If you don't know what you want to do for 8 hours a day for the next 40 years, how do you know who you want to spend everyday with until the day you die? This problem is even more compounded when the example is, as you stated, two people who have never dated anyone else. I feel that people should explore what is out there first before jumping into a decision, know all your options. Why would someone want to marry the first person the date, most likely from the same town, when there are 3 billion+ women or men out there from all over the world. It comes down to one factor in my mind, immaturity. They are too immature to understand what else is out there, and most importantly the consequences of rash decisions.
I could write more but I'm tired of this topic. Too frustrating for me to get into these peoples minds.
I'm huge in Europe