So then how long would it take until this amendment got tossed too?
I don't think this is for the Federal Government to legislate on....
If you mean "tossed" by the courts, that's impossible. The courts cannot declare the Constitution unconstitutional.
[Though I wouldn't put it behind Sandra Day O'Conner to try.]
the SCOTUS can't toss a constitutional amendment out.
Normally, in personal family matters, that should be true. However, through the Full Faith and Credit Clause of the Constitution, the courts will impose one definition of marriage on all the states. There is no way around it.
As a practical matter, it wouldn't work to have one state with homosexual marriage, another with child marriage, and another with legalized polygamy. Remember, Utah was required to step in line with the national understanding of marriage before it was allowed statehood in 1894.
The idea behind the Federal Marriage Amendment is to allow the people, not the courts, to decide what constitutes marriage.
Note that constitutional amendments aren't federal legislation; they require approval from both the Congress and the states. They are therefore a good compromise.