In his book, Why Marriage Matters: America, Equality, and Gay People’s Right to Marry, civil rights attorney, Evan Wolfson describes several reasons why civil unions are no substitute for marriage, and not the right answer for couples, communities, and our country:

• One of the main protections that come with marriage is the word marriage, and the security, clarity, and dignity it brings to families. To be denied the vocabulary of marriage and its meaningful, resonant, and readily understood statement of love and commitment – and instead, have to fumble for ten documents, an explanation of a new term that doesn’t even have a verb, and, possibly, a lawyer just to protect your family in a time of crisis – is not fair and not equal.
• Civil unions are good, but limited, and do not provide the full range of protection for families. There is only one system in our country that protects families no matter where they live or travel; it’s called marriage. Civil unions do not provide the 1,138 federal incidents of marriage, or assure security and equal treatment from state to state.
• Civil unions are a product of the work to win marriage itself; we don’t get even civil unions by asking for civil unions. Support for civil unions represent a place-holder in people’s thinking as they grapple with the need to end discrimination against gay people, same-sex couples, and our kids. Running away from a discussion of how the denial of marriage harms families, and failing to defuse this hot-button, undercuts the reachable middle’s ability to rise to fairness.
• The opponents of equality are against civil union as well as marriage, as shown by the anti-gay amendments being pushed state by state and in Congress by right-wing groups. These attack measures would deny the freedom to marry, but also civil union, domestic partnership, and any other bit of protection, large or small. Separate and unequal "compromises" satisfy no one, and legislators who capitulate on questions of fundamental fairness and basic rights buy no one off, gain no peace, spare the state no debate, avoid no primary challenges, but rather just fall short on all sides. If we are going to have to fight anyway, why not fight for what we fully deserve? In fact, authenticity and leadership actually help politicians guide the public to the right result. Consider: In Vermont, where legislators created civil union rather than ended marriage discrimination, a right-wing firestorm followed anyway, with hateful attack ads across the state, primary challenges, and electoral turbulence. By contrast, in Massachusetts, every single legislator who supported marriage equality won reelection and some of the anti's were defeated, because the public had a chance to see leadership, hear the case, and, most important, witness with their own eyes that when same-sex couples married, they didn't use up the marriage licenses and the sky didn't fall.

I have my own copy of Atty. Wolfson’s book, and I really think that this book is a wonderful resource, as well as the website.  Read more by visiting:  http://www.freedomtomarry.org .  There is a link that will take you to the Purchase page of the Freedom to Marry website.