Marriage has mostly been the province of the states. Every state has regulations regarding marriage.
The Federal Government refused to admit Utah to the Union unless the Mormon Church rejected plural marriages. It is pretty certain that Utah did not pass any laws declaring children born of second, third or ninth wives as being illegitimate.
One by one, the states will legalize same sex marriages. Perhaps some will hold out longer than others, as Arizona did over the MLK holiday. But I would estimate that by 2040, gay marriages will be recognized everywhere.
The Feds will still have to declare that the IRS marriage deduction can apply to gay marriages.
There is no chance of a constitutional amendment banning gay marriages. Every year, more of those old fogies who disapprove will die, and every year their children and grandchildren will be more tolerant, the same as occurred with interracial marriages and segregation.