Do we have any evidence that someone like Lucius Malfoy would not be about as homophobic as the average medieval baron, of the sort who would teach his son that raping uppity peasant girls with impunity is one of the bennies that comes with "good breeding?" Or that, say, Seamus Finnigan wouldn't have the same kind of teen-boy homophobia/bullying reaction that's fairly common in our world?

I don't think we do. Apart from the the recent chapter, of course. Everything prior to that would suggest a mild tradition of homophobia would be likely. It would be extremely surprising if there wasn't a bullying reaction of some kind. Children require very little excuse to bully someone atypical!