If the age of consent is 18 (and I don't think that is true across all the States), then 17 is illegal and should be illegal no matter what the age of the partner is.
I am sure a lot of people don't myself included. We are talking about the law and minors though. Society sets that "consensual" age limit. I am sure everyone has their age "line in the sand" so to speak.
But most of the time the law has provisions for edge cases, such as the Romeo and Juliet laws that exist in many states (laws that override the age of consent when the age difference is small).
Some jurisdictions disagree. Personally, I think it's a lot different for a couple who barely straddle the consent age to be having sex, than for one of them to be 50.
Why, exactly?
What if the partner is 18?
What if the partner is 35?