A wedding Arch is simply a time when all the guests that the bride & groom have been trying to avoid all day (and evening) finally get to shake hands with the groom and snog the bride
When the Bride & Groom are about to depart the Wedding Reception (hopefully together...but, you never know...) You call for all the audience to form two lines, facing each other, about 6ft apart, all the ladies in one line, all the fellas in the other line. Depending on the shape/size of the room, the lines normally start at the dancefloor and run out to the fire exit/room exit.
You then get the happy(?) couple to start at the Dancefloor end of the lines and they spend a few seconds thanking everyone for being their, hoping they enjoyed themselves....anything other than "I hope your tiny wedding present is something better than it looks when we unwrap it..."...
During the several minutes that this takes, you play some rousing music to stop the 70 couples who are standing in line, but not conversing with the bride and groom, from getting bored.
Depending on the couples arrangements, the whole audience except for the compleely plastered ones, will follow the couple out to the car park, for that age old tradition of "Will the grooms car start, even after all those things that we did to it...?"
In some rooms, two long lines just are not possible/practicle...therefore, a huge circle of guests could be made on the dance floor, you then send the bride and groom off in opposite directions around the circle...resist the urge to swap the background music for the Hokey Cokey....

Then again....