Protection of heterosexual marriage is simply the state regulating and protecting the unique institution that forms an orderly community and benefits the future of the society.
Communities and nations survive where its participants act in accord with what benefits the whole, not the few. Some will argue that the state has no "right" to prevent a same-sex pair from forming a union. To many, the very idea of the common good violates "personal rights." But there is no "right" to a sexual relationship. In truth, it is merely a personal choice—not a right—that is circumscribed.
This principle is illustrated by proposing an analogy to traffic laws. Suppose each driver were permitted to set his own rules of the road—what would happen? Within hours roads would be strewn with crashed cars and injured people. The resulting traffic jams delay others from reaching work, school, or doctor’s appointments.
The point is that such laws are enacted for the safety of both the individual and the public at large and serve the common good of the citizens. The common good means some will drive slower than their preference, stop where they think they should be permitted to proceed if left to their own accord, and park only where designated rather than anywhere they choose. The simple truth is no one objects to elevating the common good over individual preference for the sake of orderly traffic and public safety.
The question is not about denying individual rights but about promoting the good of the whole society.