This absolutely depends on the time period and location. This is not a good blanket statement to use for all of ancient greece. And even when looked down upon it was a very common occurrence anyway.
Athens had laws against adultery going as far back as Draco which is essentially since writen law was first codified at 800BC. Laws against adultery existed since the beginning of writen law. Any periods beyond that and you're into oral law. What period was adultery considered acceptable?
Which is specifically what Plato argues in the Symposium - yet, he also doesn't refute that they were lovers, and takes it for granted that they were. The symposium specifically states that in this case, Patroclus is the eromenos as a result of their hierarchical positions, rather than age.
The issue here is that greek has multiple different types of love and its not even entirely unlikely that plato (the guy who invented pla...