From the article: "...fans are writing and ask all these questions, 'I'm bullied in school... I'm afraid to come out'. They say to me, 'Could Luke be gay?' I'd say it is meant to be interpreted by the viewer... If you think Luke is gay, of course he is."
Bless you, Mark Hamill. Not only are you a compassionate human being, but one who understands that what the story means to the reader/viewer is as genuine as what it means to the story's creator(s).
Edited to expand:
I shared this on my Facebook author page as well, and someone commented, "Umm, except that is not what Mark Hamill said in the article. Thinking something is true doesn't make it true."
My answer, and further thoughts:
The headline does make it sound more definitive than it's meant to be, but I directly quoted almost everything he actually said in the article. They also include a tweet from him, in which he says, "Luke is whatever the audience wants him to be, so you can decide for yourself."
Since Luke's a fictional character whose sexuality isn't directly addressed in canon, there isn't really a "true" or "false" on the question. And mainly what I'm commending is Hamill giving hope and validation to kids whose families are failing on that job.
I think it's fine and good for the writer to say publicly, "To me, the character is this, that, and the other," in addition to whatever is already established in canon. But I think it's better still for them to add, "But if you have head-canon in which he or she is something else, and you love this idea, then that's fine too, because that makes the story meaningful in a new way."
Obviously no one wants their story to be the inspiration for a murder or anything--e.g., John Lennon's murderer being obsessed with Catcher in the Rye. I wouldn't go so far as to say, "If you think this book is saying, 'Go out and kill people,' then that's valid!" I'm talking, obviously, about head-canon that doesn't hurt anyone else even if it's fancifully different from established canon. (And in any case, I rather suspect that even if Catcher in the Rye hadn't existed, Chapman would have still had serious issues and simply named some other justification for them.)