That is only because we no longer remember the concept of "sacred". Marriage is considered sacred. Things are made sacred by "setting them apart".
On a nonreligious scale, we create things sacred in our lives by such acts as having a favorite chair or a favorite coffee cup. These objects have been set aside by members of the household in a way to honor the person's wishes to keep his favorite chair or favorite coffee cup "set aside" for the sole use of one person.
On a spiritual sense we set aside buildings and spaces within buildings for worship. These areas have been set aside in space and time for one purpose and that makes them become sacred for the people involved.
Since the earliest times we have "set aside" certain relationships and have given them honor. For example the relationship of the individual with God creates the desire and in some cases the obligation for the person to "set aside" or make a portion of his time sacred for God. This is sometimes called Sabbath, prayer time etc.
Other relationships that have been "set aside" by societal and religious norms include the parents and their children. An outward sign of the relationship of the father and his daughter is acted out in the marriage ceremony when the father gives the daughter away. What has been set aside for the father is transfered to the new husband.
Finally, the relationship of a man and a woman who have pledged a lifetime commitment is a relationship that we have "set aside" and made sacred.
Things stay sacred while they remain set aside. If the persons favorite chair and coffee cup is being constantly used by anyone and everyone then it is no longer sacred. When the bride picks any random person to escort her down the aisle, the sacredness of the event is missing. And when marriage has no entry barriers it no longer remains an arrangement that sets aside a relationship and thereby making it sacred.
What some people rightly see is a trend that will follow from removing the male/female requirement which makes marriage more separate from other relationships. This will start the beginning of ending the sacredness of this relationship.
The irony is the gays who wish to have acceptance by having their relationship considered sacred will end up destroying the meaning of marriage and losing what they want to acquire.
Hardly. It doesn't just endanger the institution of marriage, it is the second step in eliminating it as a cultural institution.
Redefining marriage to include homosexual couples does several things.
First of all it equates homosexuality and heterosexuality as morally equal and gives the government stamp of approval to same. Logically following that will be the teaching in public school that the two are morally equivalent. Of course they aren't but once the states imprimatur is put on homosexual "marriage", public schools and institutions will be required by law to preach just that.
I, of course, oppose any such morally relative notion and will fight it tooth and nail.
Next up will be hate laws forbidding the condemnation of what the government has put its stamp of approval on. In other words, preaching from the altar that homosexuality is a sinful act will become verboten.
The countries that have equated homo and hetero have a history of increasing out of wedlock birth rates and less traditional marriage. More out of wedlock birthrates result in more government, not less. The statistics are quite clear that children raised in families with one Mother and one Father are less of a burden on the public at large.
What we have learned form the liberalization of divorce law is that Fathers lose rights, children suffer, the government expands and the rest of us get to fund it all.
That public policy on the institution of marriage has resulted in more of the above and damage to the institution. Using that damage to the institution for more bad public policy that will further damage the institution is foolhardy and illogical.