It is very legally relevant that she was above the age of giving consent but not an adult in the eyes of the law. Which opens up a completely separate question of reduced age of consent laws even make any legal sense, but that's a different discussion.
It would still be really fucking creepy if she was 18, but there's certainly a difference. You're right that it's an arbitrary number, but that doesn't change that we've arbitrarily decided that someone is a minor until they're 18. An adult would understand that something being arbitrary does not make it irrelevant or something to dismiss.
Frankly, significant age differences are really fucking creepy. It still says something however if you're a grown ass adult and you're dating someone who's a minor above the age of consent.