I spoke with a Rabbi in Far Rockaway NY who indicated that in the frum world civil marriage license is not required. Just the religious ketubah is necessary?
closed as not a real question by Seth J, Isaac Moses♦, msh210♦ Jan 28 '13 at 17:39
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Not only the k'suva is necessary: there's more to a marriage, in Judaism, than that. But it's true that you don't, according to Judaism, need a New-York-State-legal marriage to be considered married. Moreover, religious Jewish societal norms are such that religious Jews will generally consider you to be married if you are married according to Judaism and not the state. However, a state marriage is necessary for other non-Judaism-related purposes, which may (I don't know) include some tax breaks, inheritance issues, custody issues in case of divorce, etc. My understanding  is that most Jews (in the United States, anyway) who are married according to Judaism are also married legally.