We cannot make a determination regarding whether something is good or better simply for being young or old. However, we can make a determination based on morality rather than age.

Something which is immoral can be described as something which causes another person (or entity) some form of harm. This is the standard around the world, shared by atheists and most or all modern religions. The "golden rule" mentality is shared my many religions.

If immorality is defined as harm, then something which is moral can be defined as something which does not harm.

In the example of gay marriage, it is prudent to determine the morality of each stance based on the amount of harm inflicted by each opposing group having their way, and in this case, it is very clear. The legalization of gay marriage will have no general effects on the groups that oppose gay marriage, will not impose anything on their belief system, and will not change their lives in any measurable way, but the outlawing of gay marriage presents a significant burden, and therefore harm, upon those in the gay community which wish to be married.

Therefore, being that harm is imposed upon gay individuals whom wish to be married, and that it imposes no harm upon individuals whom do not wish for gay couples to be allowed to marry, and given that the individuals who do not wish for gay couples to be married impose their views on the gay community, the persons who are against gay marriage are causing harm to the gay community and are thereby holding an immoral view.

The argument "I can't believe..." is not substantive, but it is valid. Its validity rises from the idea that, given that a view is seen as immoral and that many of the members of that particular society agree regarding its immorality and have for some time, that view is no longer acceptable. Social acceptability, while not the standard for proof, is a standard by which one may measure the suitability of another as an associate.