My UMC pastor has refused to even do weddings for years due to this. I had one friend ask me about doing one and I told them I would only use the Book of Common Prayer with no changes.
My denomination sometime in recent years "made it constitutional" that marriage was between one man and one woman, in order to protect clergy and churches from possible legal problems.
I know a UMC employee that was disciplined for participating on a same-sex marriage. Their plan is to continue the discipline. His real crime was not the same-sex marriage but openly criticizing the Bishopette who applied the discipline.