The legal redefinition of marriage means the loss of freedom to express and to act according to beliefs at odds with the gay agenda; more ominously, it will grant to the state an enormous and unprecedented power for remaking society according to its own designs. Marriage properly understood is an essential prerequisite for stable, healthy families. Societies throughout history and across the world have learned this from experience, and modern sociological research, despite the barrage of propaganda to the contrary, bears it out.
While marriage opponents will no doubt point to individual A or person B as an exception, we know that, on average, children raised in traditional monogamous marriages with both father and mother do better in school, are less likely to be criminals or drug-addicted, have fewer mental and emotional problems, are less likely to commit suicide, and on and on. families are, along with organized religion, the most important "mediating institutions" between the individual and the state. Mediating institutions are groups of people large and small that help serve as a check on the government, and provide individuals with a way of influencing the state much more effectively than they can do on their own.
These independent sources of authority are essential to the preservation of liberty: without them the behemoth of the state would easily crush the lone citizen. That's why totalitarians of every stripe make the subjugation or even destruction of these institutions (especially the family and organized religion) a top priority. Giving the state the power to manipulate, redefine and hence to unmake such essential protectors of freedom must necessarily lead to an ever more powerful state, and an ever smaller place for individual liberty.
