Is it true that you cant terminate a parents rights unless you remarry and new spouse adopts your kids?

I live in TN and my fiance has 3 kids whose mother has not seen or talked to her kids since 2002 and she has not paid any child support. The state can not find her and he was told the only way for him to terminate her rights was for us to get married and me to adopt them. I thought he could get her for abandonment .