What are some instances when a long standing significant custom has
been abandoned by a society? I need an answer that would weaken the
argument for history's proof that homosexual marriage is not practical
since it has been practically nonexistent until recently.

Slavery was a longstanding significant custom that was abandoned by many societies.

as was denying women the vote.

Excellent examples, but for a moment, let's look at Miltonii's
statement unrelated to the subject of homosexual marriage.

"I need an answer that would weaken the argument for history's proof
that XYZ is not practical since it has been practically nonexistent
until recently."

In the case of the above examples, the statement seems to be valid,
but basic logic does not allow a generalization:  because it was true
with these examples, one cannot make any assumption that it must be
true for any or all other examples.

During the twentieth century, Communism came and went in the course of
less than three generations.  It was certainly "practically
nonexistent" theretofore, but that was no assurance of its permanent
establishment as a practical and viable social/governmental form.

Socialism, in the milder forms practiced in Western democracies, and
"social justice" were also previously nonexistent, and during most of
our lives have seemed to work, but again, there are serious questions
now about their viability for the future:  the ability to finance
programs; the effects on society and the economy.
http://www.monthlyreview.org/0202gindin.htm

Of course, the arguments go both ways on these topics, but two or
three generations is not long enough to demonstrate that they are also
not just experiments.  And, since very few of us have experience with
any other social system, for us, these forms of socialism have the
status of "long standing significant custom."

It is simply too early to tell.  

History can only be interpreted in hindsight, it cannot predict the
future.  There are some parallels, many things being similar: one can
make assumptions about what will become of despots.
An entirely new concept, however  - communism or homosexual marriage -
 that has no historical parallel, defies prediction.

Suggest that you use these as a starting point on the history of same
sex marriages or unions. Depends whether you mean a Christian
marriage.
http://en.wikipedia.org/wiki/History_of_same-sex_unions
http://www.amazon.com/exec/obidos/tg/detail/-/0679751645?v=glance

Same sex 'marriages' are extremely viable.

In the UK we had a rush of elderly females who wanted to form mutual alliances.

Excuse me .... but did I see answerfinder making a comment?

Or am I dreaming?

Isn't it funny how answerfinder always disappears when Tony Blair goes
on holiday and then he reappears shortly after his return?

I know now exactly how Lois felt when she finally figured out Superman's identity.

Gotcher Tony!

Re:  Answerfinder
I retract my comment elsewhere.  He has not been monitoring CCTVs of
Waterloo Station or the Oval; He is a regular substitute for one of PM
TB's gorillas who is too uncouth to take abroad.  (TB could never do
what AF does here  - even with the help of Cabinet staff.)


Milton, my apologies for this off-track dialogue to your serious
question.  Answerfinder-ga is a highly esteemed Researcher  - as his
comment demonstrates - whose recent absence has elsewhere been the
subject of jocular speculation.

I hope you get some more responses to your question.
Regards, Myoarin

I very much appreciate all your comments. They are helpful in my
deciding more clearly how I stand on the gay marriage issue. Thanks.