Here is an excellent post by Nita on the impact children have on a marriage and if they can actually contribute to happiness in a marriage. It is an excellently researched and well-written article and explores different facets of the husband-wife relationship. Personally, I think children play and important role in marriages. As Nita says, much depends on the personal preference of the couple.
But in India, couples face a different kind of problem altogether. First, they really do not have a choice in having children. They are expected to have one, at least within a couple of years of the wedding. Those who don't have kids, either by choice or otherwise are often stigmatised and humiliated by family, friends and by society at large. It is true that things are changing. But, not to the extent that society quietly accepts and embraces those who choose not to procreate. Even in urban centres, the pressure on a married couple to have children is quite high.
If a couple does have children to shut society up, they end up being resentful of the situation they are in. Such an attitude helps nobody: not the children, who end up feeling unwanted and neglected, not the couple who suffocate within the confines that society has imposed on them, and not the family, which, by extension suffers too. As Nita points out, several surveys have argued that children lead to a decline in marital happiness, but "happiness" is an emotion that cannot be measured on a scale of one to ten.
Children undoubtedly influence marital happiness. But what is more important for a successful marriage, or indeed, any relationship, is the willingness to listen. I find that the better I communicate with people around me, the better my relationship with them becomes. If that is true of friendship or of professional relationships, it should be equally true of marriages. As long as the couple is willing to sit down and sort out any issues they might have, a happy marriage is impossible. Other, equally important factors such as fidelity, compatibility and understanding are essential ingredients for a happy marriage.
To sum up, a happy marriage can only happen if both partners are willing to make it work. Resentment and frustration are the main enemies of a happy marriage. Children will only make an already happy marriage even happier, just as they will worsen the quality of life in an already-bad marriage.