Well-intended, but wrong for many reasons.
The Declaration is in no way a legally binding document. This is not to negate any sentiment contained within it.
But that's the point, they are sentiments (based of course, upon the great principles and thinking of the Enlightenment, especially as conceived by our Founders).
The real no-nonsense and enduring phraseology is found in the Constitution. Note that in this utterly realistic and drop-dead serious legal document, the phrase is more properly expressed as "life, liberty, and property" (emphasis mine).
One thing that burned many Brit butts, and burns many white Liberal butts today (or would if they were smart and/or interested enough to ever read the documents) is the equation of "property" with "the pursuit of happiness."
The upshot: the government is to encourage an environment in which people can pursue their individual and diverse ends, and find meaning and fulfillment on their own terms, without dictatorial interference from economic, political, and social planners in government.
Of course, marriage rights are arguably bound up in this prescription, especially in our age. But in no way is the government to provide or ensure happiness.
All questions of gay marriage, therefore, must be separated from emotional pleading. The State does not and must not express any interest whatever in whether a couple seeking a marriage license is in love, or wants to be happy, or anything of the kind.
I cringe every time I see someone in media crying and sobbing about gay marriage, saying "It's all about love! I have the right to love!" I cringe even more when I see an elected official doing the same thing.
This kind of cloying emotional display may be satisfying in the short-term, but it is ultimately counterproductive and even dangerous to liberty. Once you allow the government to become the dispensary for all happiness, contentment, and fulfillment, you open the door to tyranny and authoritarianism.
So let's dump the emotion, and get down on the hardcore, rock-solid meat: the Constitution. Marriage is not about love, ironically enough - it's about law.