Oh, sexuality definitely has effects on identity and all the rest in Rowling's writing - as long as it's a heterosexual one. Hetero characters are bound to have all sorts of love affairs mentioned either in detail or in fleeting (well, except the ones who don't have any personal life at all, and actually no character either, like, say, McGonagall.)

But I disagree with you on the "all was needed was one sentence" bit. Writing doesn't work that way, certainly not writing about a socially problematic issue such as homosexuality. It would have taken some building and elaborating. Not much, but certainly more than one sentence.

And oh, it wouldn't have ruined the sales. I'd hazard a guess that sales would have ROCKETED once the rumor was out that Rowling has outed one of her major characters.