The objection is usually taking advantage of a trusted position or a position of relative power.
In almost all other countries in the free world, there is no such thing as 'statutory rape' only legal age which is usually around 15 years.
This says a lot about societies view on sexuality though. A man having sex with a teenage girl is a predator, a woman having sex with a teenage boy is 'nice'.
It's a recent thing too. If you go back just 20 years or more, there are plenty of mainstream movies where middle aged men have affairs with teenage girls. I recall one where Michael Caine bangs his pal's teenage daughter on vacation.
Blame It On Rio was the name. The lass with fine tits was 17 years old at the time of the shoot.