I've checked AR 670-1 (Army Reg) and found that the beret is worn with the dress green uniform before retreat. I can't find anything that says whether the beret is worn (by the groom) for a midday wedding. Any guidance would be greatly appreciated.

Personally, I think wearing a hat would be rude inside a church or during a ceremony. Truthfully though, I know the military has their own standards which should be respected... sorry I can't help more, but is there a senior officer you could ask?