They've said in tonight's episode that Mark and Natasha's children are illegitimate.
This is wrong.
Quoting from Wikipedia:
In both canon and civil law, the offspring of putative marriages are legitimate.
A
putative marriage is an apparently valid marriage, entered into in good
faith on the part of at least one of the partners, but that is legally
invalid due to a technical impediment, such as a preexistent marriage
on the part of one of the partners.