Corporations, especially old school blue-chip ones, will generally follow the path of least resistance to profits.
When GLAAD's members pay them to express positions in support of gay rights, is that morally questionable? Or is it only morally questionable when they express opinions that you do not like?
I suspect the conversation between GLAAD, AT&T, and their membership will go something like this:
Members: You're losing $500K per year
GLAAD: AT&T can you match this?
AT&T: Will $600K suffice?
GLAAD: OK.Disbelief my ass! "Fiduciary duty" is not an excuse for corporate raping and pillaging of a society (which, in effect, is what this is). Any student of economics who understands the concept of Pareto efficiency should understand that any action which causes an overall detriment to the economy or the society at large cannot lead to sustainable profits for the company involved. Sure, you can make a lot of short-term gains walking down the street and robbing everyone you see, but it's not a sound formula for long-term profits.
Fiduciary duty is no excuse here. What these companies have done is reprehensible on its face and it is a shame that it is not outright illegal! (Consequently, before anyone cries "First Amendment!", I would add that this type of situation is also a good reason why corporations should not enjoy the same status as individuals under the law.)