If even within the context of a simple thank you to a gay couple, one is necessarily implying their approval, then realize that implicature has no neutral ground. You’re suggesting that in the absence of being able to make some qualifying statement that would express your disapproval, something that would obviously be inappropriate in this situation, that the best alternative is to just omit information. Omission in this case is a form of deception, meant to distort or hide the complete truth. “Oh, maybe they’re just good friends or business partners.” And there is nothing neutral about that. It’s simply another way of expressing disapproval.
Is there no space in our society free from the gay marriage debate? When someone does a kind a generous thing, you can’t just say thank you without first qualifying that statement?
Look at the message you’re sending to this gay couple. You’re saying that in every aspect of their lives, even in their charitable endeavors, that when you look at them, what you see first and foremost is “grave sin” and must first acknowledge that because everything else is secondary: “You’re an abomination, and oh yeah, thanks for the donation.”