When someone exploits positive discrimination to get their way, and points to a personal trait of theres as a fault which somehow justifies their actions or inability to do something they'd ordinarily be expected to do.
Also used to guilt trip people into taking back something they've said on the grounds of them being insensitive.
Usually just enforces negative stereotypes and is extremely detrimental to the cases of most gay people simply wanting to be treated like normal human beings.
"Oh, sorry son. I'll do it. Thanks for playing the gay card! I nearly forgot that I'm supposed to treat you like an imbecile for something totally irrelivant to your physical and mental capacity."
"What the fuck are you on about, boy?"
"That really hurts my feelings. I have special needs! It's not easy growing up gay, you know?"
"Erh, don't play the gay card you stupid brat."
"What the hell did you do that for? You just killed 40,000 people!"
"I'm gay! How could I know any better? It's your fault, you straight people have made me unknowledgable through your discrimination."
Such people and there are many, must have previously believed that their sexual orientation may have been a hinderance in their careers. In reality, it hardly makes any difference to the general public who aren't that bothered. But continual reference to it gets boring.