I saw that and thought, "C'mon, Harry! They'd have to be morons to try to get that through! I'm sure there's more to it."

There is! Just not in a good way, according to Snopes: https://www.snopes.com/fact-check/tennessee-law-child-marriage/

Apparently the bill was originally intended for its anti-LGBTQ+ marriage "benefits" — the no age limit thing was just a happy extra bonus. The bill's sponsor is aware of the age loophole but sees no need to fix it.