In normal social situations, I see no problem with May-December relationships as long as both parties are consenting, mature adults.
I had a big problem with it in jw fundieland. I didn't know of healthy relationships like this. There was a huge tendency for men to look for someone much younger than them, if the man hadn't married the millisecond he got out of high school.
The relationships I was aware of were icky. The girls were super immature, as you can imagine when someone is deprived of normal life experiences that help you mature. They had dated little or not at all, and once they started dating an older man, they were engaged approximately 3 seconds later. They were swept off their feet with the fantasy of a ready-made husband who was spiritually and financially secure. This mate was fully ready to be her head so he could teach her about life and take care of her.
I am paraphrasing actual statements young ladies said to me back then.
My flesh crawls when I think about it.
It's a real thing in other fundie cults too. https://www.google.com/search?q=age+of+marriage+in+cults&ie=utf-8&oe=utf-8