It is a felony because the legislature made it a felony. That is all the law means, ie it was passed by a legislature.
Now I understand your point, but consider the continuum:
1. A person has sex with an 18 year old they have no relationship with.
2. A person has sex with an 18 year old employee.
3. A teacher has sex with an 18 year old student.
4. A prison guard has sex with an 18 year old prisoner.
To me 4 is clearly a coercive situation and 1 is no one’s business but the parties involved. So the question is should 2 or 3 be a be a crime?
None of your examples should be a crime. Of course the legislature of a State can make it a crime, but it does have to meet Constitutional merits which I don’t think they do.
Making sex a felony under specific circumstances but not regarding age or power seems to be arbitrary to me and simply up to the particular State. That alone should make you think about it.
It is only a felony (if State law says so) until someone brings the case to the Supreme Court and it is overturned (Unfortunately way too many laws have to go that route to be overturned). That is our system, slow but eventually seems to work things out accurately (usually)...
Logic does seem to me that any felony should pertain to an action - not an action if...
Of course I could and am often wrong, but logic would seem to support me...