On the most basic level, Death in Venice is about an older man who falls in love (or rather, lust) with a fourteen-year-old boy. That's what we call pedophilia—and it was no less shocking at the time of the novella's publication. But if we read carefully, we find out that Aschenbach's infatuation has symbolic importance, as well as shock value.
Specifically, we're talking about the following brief hint in Chapter 5, when Aschenbach wonders what other people might think about his pedophilic problems. Check it out:
He too had served; he too, like so many of them, had been soldier and warrior, for art was war, a grueling struggle that people these days were not up to for long. A life of self-domination, of "despites," a grim, dogged, abstemious life he had shaped into the emblem of a frail heroism for the times—might he not call it manly, might he not call it brave? Besides, he had the feeling that the eros which had taken possession of him was in a way singularly appropriate and suited to such a life. Had it not been held in particular esteem amongst the bravest of nations? (5.11)
Let's start with the last two sentences. What are these "bravest of nations" that Aschenbach mentions? Well, not surprisingly, among them is Ancient Greece. It's well known that Greeks condoned relationships between older men and young boys, something that scholars call pederasty to distinguish it from pedophilia, which is the crime as we know it today. Pederasty plays a role in Homer's Iliad and in Plato's Phaedrus, for instance.
Now, if we look at the beginning of this passage, we understand the logic of Aschenbach's comparison. He makes sense of his attraction to Tadzio by imagining himself metaphorically as a Greek "soldier and warrior," who leads a disciplined, "abstemious" existence, struggling to make art in an age when "people […] were not up to [it] for long." Those warriors practiced pederasty, so why can't he? Right? Right? Or so Aschenbach insists, anyway.
Hold up, though: Death in Venice is not celebrating pedophilia. Here, as elsewhere, the novella is using the example of the ancient Greek ideal in order to reveal its apparent perversity. As with Mann's use of mythological imagery (read up on that elsewhere in this "Symbols" section), ancient Greek imagery enters into Death in Venice ironically, showing the way its idealism conceals some pretty disturbing things. So Aschenbach can call his desire pederasty all he wants—Mann makes sure we know what it truly is.