But it was widely assumed that they were joined in a legal US ceremony by the time Obama was born.
I'm not so sure that's true. And the real BC may reflect that.
While the ceremony may have been “officially” handled, it was not a valid marriage because of the dual marriage.
Same as the transexual who is claiming to be the legal wife of the dead firefighter in Texas, except that he was born a dude and cannot marry another dude in the state, regardless of what the paperwork “claims”.
Falsified paperwork gets approved but does not make something legal.