Nothing in the bible states that the marriage of two homosexuals is unacceptable either. It condemns homosexual behavior (which I'm told isn't limited to the confines of marriage). The sexual and interpersonal behaviors of atheists who are married are also unlikely to meet biblical standards. Why put his "creative soul" into celebrating the latter? 

Or to use a different example, if I were a divorcee getting remarried, you could make a biblical case for the sinfulness of the "event" of my new marriage. But again, I doubt he questions every heterosexual couple who want to purchase a cake for their wedding. It's likely that he does find homosexuals to be "worse" than other sinners, but that's not biblically supported. It's cherry-picking.

The event of marriage can't be separated from the people involved, and as cake shop he's in the business of events. But if it's only events, not people that are a problem, let's remove that for a minute. Would he make a cake honoring Harvey Milk?