It shouldn't have been sexy. Crabbe and Goyle were absolutely not sexy. But the way they were together like that: big, muscular bodies, thrusting roughly and yet tender with each other at the same time, had been the most incredible thing he'd ever seen. And it had fairly well solidified the issue of Neville's sexuality—which had been not so much in question, as intentionally hazy—and he was desperate to commit every thrust, every kiss, every sigh to memory.
You've made Crabbe and Golye sexy, when, as you say, they absolutely shouldn't be. Yet this passage -- this entire fic -- is undeniably hot. And funny as well:
Wizard Love: Everything you ever wanted to know about wizards who love other wizards but were afraid or too embarrassed to ask, for obvious reasons. And it's illustrated! -- Hahaha!
I really love the flow of Neville's fantasies also; his memories of the dorms and the fact that he fantasizes about being with Harry at one point. Nice job.