It’s not about pedophilia, it’s about sexual strategy.

The people complaining about it are (usually) older and bitter that being young is more desirable. If humans are born in equal proportions, then one 40 year old and one 20 year old deciding to be together means there is one 40 and one 20 year old that can not have a relationship with someone their own age. Since usually it’s an older man with a younger woman, that means a young man has to find an older woman and an older woman has to find a younger man, both of which (usually) goes against their desires.

They obviously do not like that outcome, thus they try to limit who can date who based on age range, so at least they can get someone their own age.

This behavior is not usually totally conscious.