One is taught not to enable or support sin. Providing services to a gay wedding is supporting and enabling sin. Marriage means that the State recognizes the two as sexual partners and while being gay is not in and of itself a sin, practicing gay life style and having sex IS.
And every time a Christian sells a wedding cake to a hetero couple where one of the two partners was already unfaithful before the marriage, then they are also supporting that sin as well, right?
And every time a Christian sells a wedding cake to a smoker or a drinker, they are supporting that sin as well, right?
And every time a Christian sells a wedding cake to a Jew or a Buddhist or a Hindu for a wedding, they are supporting sin, right?
And every time a Christian sells a wedding cake to an obese person, they are supporting a sin, right? For obesity is one of the seven deadly sins in the bible..
You see, using your logic, there is practically no one to whom a Christian should be selling wedding cakes.
That line of logic is just batshit crazy.