Although it is not mandatory, I would imagine that the vast majority of councils have a building they call home. These buildings have mortgage payments and/or property taxes on them that have to be paid. Some of the money that goes to making these payments comes from renting out the building when it is not being used by the council.
If a gay couple requests to use the building for a wedding or reception and is denied, well that's that. The council will be sued, the gay couple will be given the building as damages, and the council will have to start meeting in members' living rooms.
The council might actually become stronger and gain more members over such controversies, but it will be a shame to lose the property and all of the associated memories.