The Neville I wrote in Transfigurations was extrapolated from the early books, not from the kick-ass later Neville!

Given what we know about Neville from the finished series, I'm inclined to view him as somebody you'd be lucky to find in your bed -- confident, good-humored, not at all a person to be thrown by embarrassing sex misfortunes like impotence or an ill-timed fart.

Maybe he's the kind who has difficulty taking himself seriously enough for times of emotional intensity?