The milieu as described by James Davidson in 'Courtesans and Fishcakes' was far from coercive, but does describe sometimes sexually-charged mentorship relationship between younger and older men.

I would doubt you 'had' to do anything you didn't want to, so I'd be more careful how you frame the question.

I can't speak to this broad question specifically and totally, but I can say that pederasty was primarily a practice performed by ancient Athenians. Many scholars of philosophers did have relationships with their teachers, but (if I may use some wordplay) being entered wasn't a requirement for entry to a school.

It's common these days to think all the Greek men were screwing each other (many were; bisexuality was basically the norm), but men screwing young teenage boys was mostly confined to Athens and subsections of Attica.

That being said, it's worth noting that most of this pederasty sex was neither anal nor oral. It was also not vaginal (obviously). It was called intercrural sex, which is basically rubbing one's penis between the thighs of their partner, often with lubrication; it was considered less demeaning than oral or anal sex. (Intercrural sex was not limited to antiquity, nor Greece; it has been practiced through the years. It is usually practiced facing the partner, and can be performed standing up or lying down.)

Unfortunately, I cannot remember the exact name of the book from which I learned this; it was about a year and a half ago in a college course on race, class, and sexuality in the ancient world.