Not all for profit, however they're willing to sell to others that would, in your words "make them complicit in their sin", but they're cherry picking what they feel makes them complicit in their sin.
I agree that a lot of Christians cherrypick but in this case, the shop owner has a point. If an atheist woman goes to buy a dress, as long as it's a heterosexual marriage, the shop owner is not complicit in the woman's sin of atheism and there is no doctrine forbidding atheists to get married.
As I've said before, until I know more about the shop owner and her motives, I have to assume that she did not refuse to sell to gays, she refused to sell a dress for a gay wedding. It might not seem like much but it's a huge difference.