As this discussion essentially illustrates, first you need to define sex offender.
An adult and a prepubescent is pretty much a no-brainer. It's bad. Paedophiles show high rates of reoffending. Sentences need to reflect this.
With an adult and an adolescent, you need to look at the context. I'll give an example; whilst at Univesity, in a club, I was hit on by a girl. I took her to be 18-20. At University I had been dating University students as my social life was with students, so this was nothing unusual in this even though I was older than most students.
After I'd said goodbye to her (she left early (I should have figured it out then)) I was warned by a friend in the club that she was 15 (age of consent is 16 in England, just to provide context). A few days later she popped round to my place (I lived 5 minutes from where three of the town's clubs were and loads of people knew me so it was easy for her to find out where I lived), and, without being indelicate, I could have. But I didn't.
Not because it was harmful to her; she was sexually active, made it clear she had, er, designs on me, and even in school uniform (I kid you not, she popped round straight after school) didn't look like a someone in their mid teens. I just didn't feel comfortable deciding I could ignore a law I approved of as that's what they do.
But I knew she was 15; I'd never have guessed it. If someone did have sex in a situation like that, they're not a sex offender; to lump someone who was unaware (and reasonably unaware) into the same group as paedophiles is a joke.
Prosecution of an adult for sex in similar situations where there is no harm or coercement would be a joke.
But, just as some adolescents are precocious, so are some still very immature and in need of the protection offered them by law. Some people are attracted to adolsescents (the use of the term hebephile helps distinguish them from paedophiles) and if they were to use coercement or grooming techniques in pursuit of the object of their sexual attraction, then that's no joke, and completely different from the 'mature minor' examples I give above.