The short term answer is obviously to withhold the truth. You cannot speak honestly to someone when declining their business, not in the country that our United States has now become. Accept whatever diversity drivel you hear and then, at some later point that day, a week later, or the day before the wedding, plead a conflict, personal crisis, or whatever to regretfully cancel. There is a time and a place for everything, but when you are vulnerable to Gestapo tactics is not the time to explain that decent people neither celebrate perversion nor directly enable such celebrations.
The long term answer is to fix our laws. Physical violence against gays because they are gay is bad, just as bad as the legal violence against Christians that these activists are initiating. However, we have an absolute right as free people to refuse service to any potential customer for any reason or without reason. We need to reclaim that human right as a legal right. We also need to reclaim that human right as a practical right, in part by shunning those who engage is gay-activist blackmail and related abuses of the legal system. It’s time for decent people to permanently boycott those who support slavery and the use of government force to compel involuntary servitude.
You don’t have to bake their cake, it’s a contract not slavery. People can pick and choose if they want to work on a project or not.