I thought that your legal name was the name on your birth certificate.Isn't that the name you would use to get a marriage license or a passport?
An actor could legally change their name so that their stage name would then be their legal name.In that case the name change is done through a court and a new birth certificate is issued with the new name.
If people could just constantly adopt any name they felt like, as their legal name, without a legal name change, there would be no consistency in vital records that establish identity---like birth certificates, social security number, voter registration, etc.
You can call yourself by any name you wish.But that doesn't make it your "legal" name.
So, to answer your question---I think that Nicole Kidman would be Mrs Tom Whatever-his-name-is-on-his-birth-certificate.
Their childrens' birth certificates should really have the parent's legal names on them.
Why would an actor not want to use his real name on official documents?
[This message has been edited by chelseabelle (edited 01-08-2000).]