I think your presumption that individuals who oppose same-sex marriages are afraid of something is invalid. Values are not fear-based but rather are based on reasoned right and wrong, as well as appropriate and inappropriate behavior. You would not say that people are scared of alcoholics simply because they find drunken, lewd, behavior obnoxious, would you? Nor would you suggest that society should not make laws regulating this behavior simply because the individuals claim they have a genetic propensity for alcoholism, would you? Well, homosexuality is no different. Individuals who choose to engage in same-gender sex should not be granted special rights based on their sexual mis-orientation. As individuals in our society, they have the right as any other citizen to marry a person of the opposite gender. They know full well that (in same-sex relationships) they do not have the rights of a married couple, nor should they, just as a drunk has no right to drive, simply because he or she has never been in an accident.
Choosing to be homosexual does not make homosexuality a civil right. Society does not need to endorse or condone any inappropriate behavior that is unacceptable to the functioning of a civilized society. This has nothing to do with fear and eveything to do with values, family and the long-term health of children.