Every culture has norms regarding what is accepted sexual behaviour and what is not. The male homoeroticism was widely spread all over the Greek world, but the customs and expressions could vary between different parts of Greece. And as usual most of our information sources come from the city of Athens.
Greek society did not distinguish sexual desire or behavior by the gender of the participants, but rather by the role that each participant played in the sex act, that of active penetrator or passive penetrated. A man was supposed to behave like a man, that is being the active penetrator, no matter if it was vaginal, anal or oral sex. A man who let himself be penetrated or who performed oral sex (yes, the one who received oral sex was seen as the active one) could be branded as a ridiculous kinaidos ("pervert"). Of course this doesn't mean that people in ancient Greece never had anal or oral sex. Just that those kind of sexual practices were negatively charged. And it's always a bit difficult to know how reality really looked like compared to norms and ideals.