Well, to begin, the definition of marrige in the west is a union of two families (i am from the east), a union of a man and a woman. Before, the idea of having a casual relationship was something embarrasing and something that was seen as a 'disgrace' if it were to be out in the public. ( The school history textbook used in the UK, chapter life in the UK). Now, lately, casual relationships have become very common and something of 'pride'. Teenagers especially have become very fond of the idea.
However, this is the problem, one of the important reasons why marriges form is to fulfill desire. Although it is far from the most important ones. However, it is a big one. Now, at a tender age, especially at adolescenence, 'love' turns up to be a common misconception. At this age, due to hormone fluctuations in the body, there is a mix-up between love and lust. And this mix-up has been promoted over and over again from childhood, like with movies like Cinderella, now a days there are very few people in the world who have not heard the story. Later grown up novels and movies with 'adult' parts in it.
So, I know I will face a lot of critiscism, this is 'typically' what happens. I know there will always be exceptions. But this is 'generally' what happens. Now, this confusion is prolonged because the values that the older generation had about the idea of 'union', the idea that there should be a distance between boys a girls. So, this confusion becomes a fact and a simple thought, 'If I can get what I want of a wife from a girl-friend why the heck should I get married?". So as you can see many people have children with their girl-friends and there is nothig the law does to prohibit this, in most western countries.
Now, so, the man, views that, marrrige is simply a 'sticker' of a formal relation. But, this is where he forgets the commitment. The woman, who is also likely to have many casual relationships in the past thinks similarly. In the US, the average number of casual parterners for male and female is around 8.
SO, now what happens, is the responsibility is erased from the relationship and it simply becomes another casual ones.
Now children, the consequences of a break up is severe. They are not able to go through normal and health growths. So because of such a relation, a relation with little commitment, the children suffer. Now imagine this happening in hunderds of families and now imagine the consequences.
So if marrige is not taken seriously, the children suffer. If the children suffer, the next generation learn the wrong things. Without proper parental guidance, imagine what could happen.
So if there is no commitment, it is not a sinciere relationship and it ends in the next generation being 'screwed'. Marrige is how society is linked, it puts the authority of two families, and thus manu families together. If there is not commitment in a marrige, and as I have said marriges, society collapses.
Now this might be normal to you, I have seen people who have had 'it' many times without telling their parents. Infact this is a key issue in America and the UK.
Hope that answers your question.