Let me try to answer that..
Marrige is a kind or written agreeement that Two parties will live together and they will stay commited as long as this agreement is valid.
I agree that two people can stay together without marrying and they can also stay commited. Marrige is like a driving licence, You may be really good driver but still you need licence whenever you are on driver's seat.
Lets come on having kids, Our body is a comlex machine that needs to be maintained well. After 30, We need more disciplined life, balance life style and that can only be possible when we have peace at home and we have kids to put right examples. Once anybody has kids, He/she may think million times before desserting the partner/spouse and it brings the stablity in life and person tend to live routine life.
As a bachelor, we live with imaginations and we do whateve we think is right so all action are relative and we don't bother about social norms. However after getting married and having kids, we start following the social agenda and we start getting accomdating in some type of patter our parents had.
I think toughest thing to get married is find a right person and another tough part is to stay with same person for another 50-60 years..