Adding in what Gryph said, in a lot of jurisdictions in the real world, if the father is not on the birth certificate (or a different person is, for convenience's sake) then the only way for the whole thing to work out is to go through the adoption process. That gives a defining legal situation, everyone's covered up. Sometimes, even if there's no good legal reason, it happens. (Go read back on Wil Wheaton's blog where one of his stepsons asked Wil to legally adopt him, which goes into some details on the situation.)

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Geoff Depew - Darth Mephron
Haberdasher to Androids, Dark Lord of Sith Tech Support.
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