Why do some people think you should marry a woman just because of an unplanned pregnancy?

A lot of people use the argument if you feel you are man enough to get a girl pregnant then you should be man enough to take responsibility for the pregnancy, marry her and build a life together. Also they say you don't want that child to be illegitimate or born outside of wedlock. Why should a couple use marriage as a form of damage control? Getting married on these terms is a recipe for disaster in my opinion, because the marriage wasn't being built on something solid so the decision was made out of crisis and panic and not wanting people to label you as irresponsible or promiscuous. Now I'm not saying a man shouldn't step up and be a father..I'm just saying more than likely the marriage won't last so why waste everybody's time?