The point of an age of consent is to send a message to a (in Poland for example) 15-year-old that "Now the state no longer decides for you. You are old enough to choose. You have the right and the responsibility to control your own body."

When you add age-gap exceptions to the law, the message becomes "You may be old enough to choose. It depends. You sometimes have the right to control your own body, but obviously you're too stupid to consent to people of a certain age who can brainwash you."

The later is a demeaning, disempowering, counterproductive message.