Read Romans 7:3 Using Other Translations
So then if, while her husband liveth, she be married to another man, she shall be called an adulteress: but if her husband be dead, she is free from that law; so that she is no adulteress, though she be married to another man.
Accordingly, she will be called an adulteress if she lives with another man while her husband is alive. But if her husband dies, she is free from that law, and if she marries another man she is not an adulteress.
So while her husband is alive, she would be committing adultery if she married another man. But if her husband dies, she is free from that law and does not commit adultery when she remarries.