The Pronoun Problem is what happens when you have someone who isn't a good writer trying to write a sex scene between two people of the same sex. It's easy to see how it happens; you get sick of saying (for instance) 'then Draco kissed such-and-such and then Harry touched this thing, and then Draco (CENSORED)', so you naturally want to slip in a few pronouns. The Problem occurs when you lose track of who's the subject and who's the object, i.e., which 'he' you're talking about. When handled particularly badly, the Pronoun Problem can lead to the poor over-stretched canon making both characters do the action simultaneously, never a good sight for an agent with a weak stomach and an inability to suspend disbelief.