Is a marriage considered legal if not registered?

Once the marriage is licensed, solemnized, and the certificate authenticated, you are considered married. Failure to register the certificate with the county recorder is a technicality that will not invalidate the marriage. It's just that it is easier to prove you are married if you have the certificate that's been stamped by the county clerk.
There's a such thing as common law marriage which, in many states, is as legal as a registered ceremony.
You will not have access to government benefits or services.
Depends on the state. In some states just presenting yourself as married, such as opening a bank account etc... can render you legally married.
If you mean filing the marriage license or certificate - the answer is no. Once you obtain the license and go through the ceremony, you have a certain number of days in which to file the signed marriage certificate. Failure to file it with the proper authorities within that time means you are not legally married. You need to check with your local county courthouse/registrar.
In most states the person who officiates over a marriage is required to themselves register in within a certain time period. Of course this would be to avoid couples years later suddenly finding out that they are basically not married.

Even if no papers were signed at all but the person doing the marriage is licensed to perform marriage it still sticks. All it takes is a couple of witnesses to come forth and say they saw the marriage. I actually knew some people who did this drunk in a bar and there happened to be a licensed person about. The guy said they were not married as no papers were signed. The women had witnesses so it stuck in court.

Regardless as another said presenting yourself as married with financial transactions could give the possibilty as being deemed married. Even if not deemed as married enough doubt can be raised for another to get money as if they had.

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