As a feminist I do not fully agree with nor wish to support marriage as it operates in American society (and most societies) today - however I still have to admit that there are real solid reasons for women, especially young women to marry (even at young ages).
Marriage provides a certain amount of guaranteed security for young women in worst case scenarios. Of course, I am not talking about weddings here - weddings do not necessarily come with marriage and should be considered separately. All too often in the Midwest young women, sometimes those who barely have begun to approach womanhood attach themselves to a male very seriously. Not only do girls become attached to males at young ages romantically - they begin to identify themselves as an extension of that male (usually as his girlfriend or wife with little distinction between these two in their actions). While I do not consider this a healthy practice, that it happens should not be ignored or glossed over. The failure of these relationships for the individual who has begun to do this (even at the tender age of 18 or younger) can be largely traumatic. What's more, this pattern of male-identification sets up a similar pattern of dependency for the future.
When financial sharing has occurred, a marriage (be it religious or civic) is usually in her best interest. During a divorce even without practices like alimony, finances get split pretty evenly or rather fairly. Because the United States still predominately supports sexism, many women in a relationship play the role of a wife - often long before a ring is on her finger (if there ever is one) in committed relationships. She will quit working, have children, or otherwise make herself financially dependent upon her partner (a Husband verses husband). This has been the traditional male-female relationship model, which incidentally, even a large number of same sex relationships are based upon: a dominant, provider, active role and a more passive, caretaker role. Culturally the women assume the passive role and often when breaks up occur in these un-secured relationships they are left penniless and without work experience since care work and housework are largely devalued. Like divorcees of fifty years ago these unmarried wives seem to have wasted that portion of their life. The courts today provide some (if inadequate) degree of fairness and provision for women in this situation who have entered into contracts of marriage. After all, would you buy a car without signing the contract? Essentially, engaging in these practices outside marriage does just this.
In the case of children - and although I would want to avoid marriage simply due to a pregnancy, marriage once again benefits the female. A divorce involving children can ensure that a fair (fair to both parties) child support agreement be reached - and reached QUICKLY. Without marriage, unless the father has a great desire to be a part of the child's life or offer financial support it can be difficult to get secured and leave the new mother in a sort of limbo with a baby. Also, the security of a healthy marriage with children for young women provides support dealing with a pregnancy and raising children. The stability that marriage can provide when it comes to everyday routines benefits young children tremendously in giving them a sense of security and well being. Not to mention the financial benefits of marriage when children are involved - the wage gap between the sexes, unfortunately, is growing rather than diminishing and with babies and children every penny counts - being able to have two incomes coming into one household benefits children. Also, those women (or men) who desire to have the option of staying home and caring for their children are more likely to be able to do this within marriage (despite the dangers of this practices, parents should be able to choose it if it's important to them even if just for a fraction of their children's development). This stability can be achieved without marriage however - within marriage it's much easier to achieve and ensure.
While the financial benefits are compelling, there are many advantages to making the person you currently reside with or spend the majority of your time with and share life decisions with legally tied to you. Sometimes, with parents or otherwise legal guardians/next of kin in other locations in the event of emergencies (medical or mundane) it can be difficult to get approval or support in decisions. Plus, if this is the person you have chosen to partner with - one would hope they share your values and morals or at the very least can respect and remember them. I think it's important not to ignore these aspects of the creation of family relations - and often are formed without the added protection of legal marriages.
For better, or worse most of the benefits for young women to marry are with the assumption that attitudes about "till death do us part" have changed. With the divorce rate at a staggering 60% (where it has stayed for the most part since the 1970s by the way) the legal procedure of joining both lives and finances revolve around the worst case scenario. For the most part marriage is a civic and religious institution - many Juedo-Chistian religions (which prevail in the United States) require that in order for a couple to engage in condoned sexual activity they must enter into marriage. In my humble opinion, if marriage should remain a lifetime commitment this belief poses many problems: many marriages do (and rightly should) end over sexual dissatisfaction. A way to avoid this is to accept premarital sexual expression to ensure when these commitments are made everyone knows just what they're getting. Weather one is morally okay with divorce or not does not always matter when it comes down to it - and in this situation it sometimes benefits young women to marry before they take on the role of wife (and very, very rarely is it a Wife verses a wife) in order to ensure their own well being and that of any children which could result from this relationship.