Jay - people band together, business owners band together, and pretty soon there is no cake baker or candlestick maker who will bake a cake for those who are the race, religion, ethnicity that is to be shunned.  That is how segregation worked in the South - by the agreement of the majority.  

In cities and large towns there are many cake bakers.  The same is not true everywhere.  When the gay marriage issue was heating up, there was a mid sized town in which all the cake bakers had gotten together and decided none of them would sell a cake for a gay wedding.

The person who supplies the food or decorations or tables or tent for a gay wedding are not endorsing the wedding.  They are providing a good or service and doing so implies nothing about their beliefs regarding the validity of a marriage.  How many of Catholic service providers supply goods for a wedding for divorced people and think nothing of it?