I find our culture's (I can only speak for the US) overwhelming urgency to engage us in long-term monogamous relationships from the day we hit puberty (or even before) a bit perplexing, really. There are many ways to live and love, and only some of them involve spending your life romantically bonded to another.
That said, I have two main reasons for wanting to remain married to my husband.
Firstly, I love him, and with my love comes a desire to stay with him and only him (romantically). In this respect, our relationship can be seen as merely a way of categorizing our love. There's the love I have for my friends, the love I have for my mother and son, and, entirely separate from either of those, the love I have for my husband. Having an official relationship also makes providing the conditions for it much simpler. As husband and wife, we've agreed on the terms of our monogamy, separation, and etc.
Secondly, As you mentioned, family. Being in the relationship that we are, we've identified ourselves as our son's only two permanent guardians (or at least, heaven forbid, until we either die or lose custody). We raise him as the pair who conceived him, and hold the ultimate authority over how he's raised. Our marriage also entails a good many legal benefits, especially when it comes to our son, but I honestly have a difficult time keeping track of those tidbits.
To me, the companionship of marriage is one without equal comparison. Now, in my perfect world, the only definite difference between marriage and friendship would be the involvement of our sexualities. However, much to my dislike, this is hardly ever the case. My friends have never considered themselves to have much obligation to me at all, let alone the astounding sense of commitment it takes to stay married. In fact, it seems as though most of them view "friendship" as nothing more than a relationship in which two people occasionally prefer to seek each other's company. The monthly "hang out" time we share is more than satisfactory to them. Past that, we have little ties, emotional or otherwise, to one another.
My husband, on the other hand, has sworn both his love and commitment to me in the presence of his goddess, my God, and our families. He doesn't plan on bunking out the first time it rains. He's prepared to wake up to the sight of my unshowered face until he awakes no more. For us at least, this type of bond was deserving of an official stamp and an announcement to the world (or maybe just Trenton).
Also fuzzy feel-goods and some unapologetic selfishness. But mostly the muddled mess I tried to describe above.