My first thought also went to the gravestones. The combination of wedding party screening them and a vertical composition could address this. But they may also feel it's part of the natural cycle of life. The group could also be in front of the bench if you (they) prefer.

Contrast (based on the photo you posted) could be an issue. A cloudy day might actually help.

Most, but not all, wedding photos I've seen seem to show the B&G standing - I assume to best show the dress and for a less "stocky" look. But hard to imagine a hard and fast rule. Is there a reason why you can't do multiple shots (standing and sitting)? Also, I've seen chairs and benches used with shots that might include elderly family members.