Do your gay friends agree that they chose to be gay? I'm anything but PC. Maybe you're just not great at metaphors. Whether you believe people are "born gay" or not, it's not something you just pick like a religion or a car.
And even if it was, these hyper-religious blowhards will congregate in church to talk about how anyone who is not a very specific denomination of the Baptist church is going to , but you can bet they'd make a cake for a wedding occurring in any of those other churches. Even, God forbid, a Catholic wedding. They'll bake a cake for a heterosexual couple who is on their fourth marriage each. By discriminating on gay weddings, they've arbitrarily decided which weddings they refuse to offer a product for.
And there's a difference between designing a product that says something you disagree with, and designing a product for people whose race or sexual orientation is outside of your comfort zone. Nobody's asking these people to make a cake with two guys ing. They're being asked to offer the same product they offer to everyone else to a couple who are the same gender.