Sky lanterns are banned in Australia because of the risk of causing an uncontrolled fire. Even though most of them are biodegardable, they are still a pollutant, and whoever sends them up most likely won't go after it and throw it away when it does eventually land. Perhaps just the wedding couple could send one up for its symbolism, but not the whole group.
I think banning them or very tight controls make a lot of sense,
I also agree for a symbol the couple sending up a heart shaped one makes sense not so much the whole party sending them up