Islam allows it's men to take multiple wives. If this is not legal, then freedom of religion us not being upheld.
inB4: omgterrorists
inB4: STDs!
inB4: different = bad
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When DOMA was in effect, polygamy was actually possible, but not necessarily straight polygamy. Someone could get gay married in a state that recognized it; because the federal government didn't recognize that marriage, one could then get straight married in a state that didn't recognize gay marriage. Thus, one would be legally married to two people at once.