What is the punishment for the sodomites in Cantos 15-16 of Inferno?
Dante puts the sodomites at the bottom of the seventh circle of hell. That's bad. Within the seventh circle, there are three rings. In ring number 1, Dante puts people who killed for their own material gain: so you have empire-builders, gluttons, and robbing travellers on the highway. People tend to get appropraite punishment, depending on the severity of their sins. So Alexander, a mass murderer, is in the boiling blood up to his eyebrows; highway robbers are only up to their ankles.
In ring number 2, we have people who killed themselves. They are worth than people who kill others, as they broke Nature's rule of self-preservation. Yet in ring number 3 - so even further down and deeper in hell - are the sodomites. Sodomy is even worse as an offence to nature than killing yourself, or killing other people. It's a pretty extreme judgement.
How does he punish them? They have to run, together, forever, in a group, across sands which burn alight:
For hark! on yonder plain what clamours swell!
And see! in tempests roll’d, the burning sand,
Mingled with smoke, ascends the glowing sky!
The burning sand on which they run is supposed to represent their sterility: their inability to have children. It's not gay-friendly stuff.