I dont understand why Planned Parenthood or these other groups were fighting this. Seems weird as hell to me.

"But after facing opposition from the American Civil Liberties Union, the Children’s Law Center and Planned Parenthood, legislators watered down the measure, citing concerns over how a ban could restrict a child’s reproductive freedom and ability to make healthcare decisions. "

It sounds like perhaps prior versions of the draft legislation could have been used against a pregnant young woman (at age 17) removing the newly pregnant young woman her own reproductive choices. That would seem to stand in opposition to the Planned Parenthood mission.

"Planned Parenthood, an influential group in California politics and a longtime opponent to child marriage bans, stopped fighting the proposal for the first time this year."

So it sounds like the draft legislation was modified and Planned Parenthood put their support behind it and now its signed into law.