Ceremony chair etiquette?

I know that full on standing receptions are in poor taste but can someone critique my set up. Looking to get married in a public park. Will like have roughly 20 guests in attendance. The park has stationary benches (6/7 that can fit 3 people each. Problem is the benches are stationary and can’t be moved. Having a small 10/15 minute ceremony and I’m wondering if it makes sense to just allow our guests to use the benches until our officiant makes the announcement for the ceremony to begin? Is this still poor etiquette or should we rent chairs as well, keep in mind we would also need to arrange transport.