I really don't think Rowling meant to portray homosexual relationships negatively by this. Actually I don't think the gay relationship itself is "twisted and evil", just the person he had it with. If Grindelwald had been a woman, a really evil witch who killed thousands of people, and Dumbledore was once in love with her, that wouldn't have been construed as an attack on heterosexual romances - just showed that Dumbledore wasn't perfect. Even very thoughtful people, like Ms. Rowling, can't think everything through. For that matter, neither can I.