We was not an "adoptive" father.
Joseph was the LEGAL father.
If Joseph was not the legal father, then Jesus would have been an illegitimate child.
I don't know where these authors dream up this nonsense about "foster father".
Was Mary married to Joseph or not?
Well, when Mary winds up being pregnant before Joseph was ever married or had sex with her, it means she was “with child” for at least a month before Joseph decided to go through with the marriage.
If I marry someone whose unborn child is another’s, I need to adopt that child. I am legally that child’s father in all ways, and the child is “legitimate”.