The only way to understand the sexual culture of Greeks is by going back to the past. We need to first look at the relationships between men and women and then compare them to the relationship between males.
This article explains the roots of this urban myth.
It’s not the kind of relationship you have in mind. It was not specifically about sex. Some Greeks had long-lasting and loving relationships with their counterparts. However, the most common phenomenon was the intimacy between older men (erastes) and younger males (eromenos). This is also known as ‘Paiderastia’ in Latin or ‘Pederasty’ in English.
The Eromenos were submissive, while the older men dominated, teaching the boys about the accepted ways of the society. However, we haven’t gotten to the heart of the question yet. Was there some guy on guy action?
Today, that kind of relationship between men would be considered abuse. The Greeks saw it from a different perspective. However, you must understand it never happened between colleagues. Sex between grown men was thought to feminize the recipient. You wouldn’t want to be considered as ‘feminine’ in a society that looked down on females.
That said, it’s clear that the Greeks weren’t much into anal sex. If it happened, it was done in secret or with a person of low status, such as slaves. No self-respecting gentleman wanted to be associated with it.