There's just something about the sound of men moaning, grunting, and screaming in pleasure that turns me on so hard. Bonus points if the person humps animalistically while cumming. Extra bonus if their bodies spasm, twitch, and/or shake during their orgasm.
As much as I'm also into females, you see this all the time with them. These things are rarer amongst men for whatever reason.
It's more fun to watch them in such a condition when they're going solo. I'm not sure why though.