Your question is really two questions. The first--why there is such an
adverse reaction in the United States to gay marriages and civil
unions--can be answered more easily, or let's say more factually. The
second question--why gay marriage should be considered an attack on
the institution of marriage--can only be answered by reviewing the
arguments of those who make such a claim.
As for the first question, there can be little doubt why a majority of
Americans oppose marriage or (in lesser numbers) gay unions: their
religious beliefs. Several facts must be noted:
1. America is a more churchgoing nation that any other developed
country in the world. "Fully 44 percent of Americans attend church
once a week, not counting funerals, christenings and baptisms,
compared with 27 percent of people in Great Britain, 21 percent of the
French, 4 percent of Swedes and 3 percent of Japanese."
2. A rapidly growing percentage of these American churchgoers belong
to fundamentalist and evangelical denominations. It's interesting to
compare the United States to Canada. Canada is the country in the
world most like our own, yet a majority of Canadians support the right
of gays and lesbians to marry. Two demographic factors can be cited. A
smaller percentage of Canadians attend church (44% US, 38% Canada, see
the above website) and Canada has a smaller number of fundamentalist
and evangelical Christians.
As for the second part of your question, you already know I am sure
that much of the "one man, one woman" definition of marriage is based
on (arguable)biblical texts. But as for the specific fear that gay
marriage will lead to the destruction of the institution, here is one
summary of that position's reasoning:
"...pro-family individuals understand that once society exceeds the
time-tested boundaries of marriage between one man and one woman,
there is no logical stopping point. 'If we don't preserve marriage
today, we're opening the door to allowing not only homosexual
marriages but polygamy and bigamy as well. And that means that
ultimately, marriage will become meaningless,' Clark says."
This statement can be founded from the links of the web site of the
American Family Association.
I hope this sheds some light for you on why this issue is likely to so
passionately engage America this election year.