I think that marriage is a joining of families, and to ignore parents, siblings, etc., the primary people who shape who we are for our first 18+ years of life is to completely disregard the people who raised us and to give up a part of ourselves, of our history.
That being said, families shouldn't be involved in every decision. There are certainly aspects of a marriage that should be personal and private, but families should be brought in on some things. Our families know us in ways different from the ways our spouses know us, and thus can offer different perspectives. They can calm you down when you've had an argument and help guide you back to reason. They can also provide wisdom from experiences you haven't personally had. My parents, and I think the parents of most people, are truly concerned for both my and my husband's best interest. They don't meddle, but they do help make decisions because they know things we don't. When we started to invest, they provided knowledge. When we buy a house, our parents will certainly be involved--it's something they have done multiple times and we haven't. To not involve them would be stupid on our part. They also provide perspectives that you might not have. For example, when my sister was set on a horrendous name for her child, my parents were able to step and prevent my niece's nick name in school from being "cat wizz".
So while I think some things, like your sex life, should be kept private, but the big, life-altering decisions should be made with a group of those who are interested in your best interests as a couple and can bring knowledge and experience to the table. Families are often those people (provided they are a positive, healthy influence; void all of the above if they're not).