What defense? People have reasonable arguments as to why they do something, it's fair game. People try to guilt others into doing what they want, it isn't.
After you factor in local costs like labor, product cost, maintenance, landscaping, etc...twenty percent might go back directly to corporate? It's been a long time, so I have to draw on memory. Of that, maybe half a percent or less might be donated to what would be considered anti-LGBT causes (they're Christian, everything is anti-LGBT almost)? You expect people to feel the pang of regret over a few cents off their $10 weekly purchase going to an anti-LGBT cause?
I live in Georgia. I can't exist without something I'm doing going indirectly to discriminate against someone who is gay. I'm not going to feel guilty about the occasional meal there, and guilt tripping over the internet doesn't change that. Not when I've seen, known, worked, and loved the people at the store level who I know don't hold the same values as those up top. I worked there for over five years, saw a Cathy exactly once in passing. I was never pressured to live a Christian lifestyle, I never was told to discriminate against anyone, and knew personally of at least two people in my own store who were gay, who were out to the operator, and was loved equally by them as everyone else who worked there. It was an extremely positive experience in my life, and wouldn't hesitate at all to send my kids to work there, because I know they aren't going to be treated like shit.
I'm sorry the Cathys are how they are. I wish they weren't. I understand why people won't go there. I support that. My contributions more than make up what I end up sending to these causes, and if that isn't enough your opinion is noted and filed away.