Honestly, if the company heads used their own income to support this, I wouldn't care. It's their personal money. I don't agree with what they want to do, but I support their right to say it.
But the fact that the company itself is doing the supporting strikes me as quite different. Now, instead of the profit they make off selling me chicken just going into paychecks, a portion of it goes directly toward funding groups supporting homophobic legislation? That's a step too far.
I mean, really, if each chicken sandwich had a sticker on it proclaiming that 1% of all profits were donated to fight gay marriage, would people still want to eat there? (I have no idea how much is actually donated, but any portion of my purchase going directly towards this kind of thing is enough to get me to boycott)