Is the wife still considered her husband's wife, or has she become unlawful to him, if the husband denied the marriage and the pregnancy, and claimed that the marriage was merely for official papers?

If you married the man with a valid, lawful marriage and he has not divorced you, then you are under his marital authority, and the pregnancy is attributed to him and cannot be disavowed except through li'an (oath of condemnation). His denial of the marriage is not considered a divorce or annulment. If the woman claims the marriage and the man denies it, but she swears an oath, the marriage is established. It is advisable to refer such matters to the Sharia court for a ruling.