Why not? You're basically saying that it's okay for say a 17 year old to date a 15 year old, but it's not okay for a 18 year old to date a 17 year old, all because one of them is an adult and the other one isn't. Imagine two 17 year olds dating normally and one of them naturally turns 18 before the other and the barely 18 year old then gets automatically get accused of pedophilia. That's literally what you're describing.