We are all sinners, and are slaves to sin until Christ sets us free (through the work of the Holy Spirit) -- but Christians are not going to participate with someone's celebration of their sin - particularly one as grievous as slandering God's first institution after Creation: the marriage of one man to one woman.
Simply put - you cannot force a Christian to participate
Simply put - that’s exactly what the left intends to do - force Christians to participate in behaviors contrary to their faith.
There is nothing simpler to say about this issue.
If it were not otherwise, they’d just go somewhere else to get their pizza/flowers/cake.