The State (Govt) most certainly has an interest in preserving marriage, as marriage results in families. Familes are what make up villages/towns/cities/states. Without the "family", society, and to a large extent, basic human morality are thrown under a bus.
The State recognizing marriage as one man-one woman preserves that tradition which has held society together. If the State were to suddenly dissolve that, and make marriage and family whatever anyone wants it to be, society will suffer because of that experiment.
As an adoptee, I'm firmly against homosexuals adopting children. Why? Again, the liberals have created an environment of action without personal responsibility. If someone is homosexual, they physically cannot have kids...so why do we let them adopt?
Society wouldn't break down if people were held responsible for the behavior. The real slipperly slope is adding more and more government regulation to correct the liberal belief of Action without Responsibility.