The Law has not changed.
Deuteronomy 23:2 does not refer to mere illegitimacy, it refers to what is called a “mamzer.”
Again, a mamzer is not a mere bastard, but rather a product of a married woman’s adultery or incest.
Christians frequently think this because the English translation of “mamzer” is incorrectly translated “bastard.”
http://en.wikipedia.org/wiki/Mamzer
Begs the question of how it was possible to routinely know whether the child of a married woman was the result of adultery, prior to DNA testing that is.
I would think a married woman’s incest would also be adultery.