It's clear to most people that a marriage between someone 50 and someone 14 is a serious power imbalance, coercive beyond the younger person's ability to consent, and nearly everyone in our society thinks it seems gross and wrong. But as far as I can tell such marriages have not been happening in California. Prior to the new law, the consent of the parents and a judge was required.

(Personally I think a marriage between someone 50 and someone 18 still seems bad. But the way our legal system works, once someone is 18 anything goes.)

It's less clear that a marriage between a 17-year-old and a 16-year-old or between a 19-year-old and a 17-year-old is inherently problematic. If, for example, the woman of such a couple gets pregnant, being married could give helpful legal protection/advantage, since most of our laws are written around the default expectation of married parents.

According to a news article from a few years ago, groups opposing a previous ban on child marriages in California included the ACLU (who claimed that it "unnecessarily and unduly intrudes on the fundamental right of marriage without sufficient cause") and Planned Parenthood (who said the law should "not impede on the reproductive rights of minors and their ability to decide what is best for them, their health and their lives").