I don't think that the idea was "Why would you stand up for a gay man if you're not gay?". It was more along the lines of "Since you are not a representative of this group, what would give you the motivation to pick up THIS particular cup and follow it through to completion?" There are lots of other injustices that he might be a representative member of and could have campaigned against.

I'm really happy that he picked it up on principle. In a way, it's a lot more pure that someone _not_ gay spearheaded the effort so there is no appearance of a conflict of interest (rational or irrational). Turing was the man, and I'm glad Britain recognizes that now.