Because people often make sweeping generalizations about age gap relationships without applying context and nuance..... Either they're all bad and creepy, or they're all fine and dandy.

So when somebody comes in saying "the age gap isn't the issue" when in a case like this the age gap absolutely is an issue, it becomes necessary to point out the differences between grown people grooming inexperienced teens, and two fully grown adults meeting each other as adults and being experienced and mature enough to be on the same page.

This is why people point out that an age gap between a 30/40 yr old is vastly different than an age gap between a 19/29 yr old.