I lived in Charleston, South Carolina and visited some of the plantation houses that are still quite lovely historical sites because the considerable amount of money required to keep them up comes from tours and special events like weddings. They are all privately owned, so if they want to hold weddings there they can. Many of the houses have special tours and even rebuilt slave cabins and other educational material about slavery, and some do not.

Where your argument fails is that these Antebellum houses were somehow uniquely bad for slaves, versus the entire rest of the South. Charleston is often called the Holy City because of the number of churches on the penninsula, but gee willikers who built most of those churches? Slaves had a hand in building every historical building in the state. That you could find anywhere in South Carolina, or Virginia, or Alabama untouched by slavery is simply impossible.

If you live in the South you could talk to friends and family about the appropriateness of celebrating a wedding at a plantation house, or refuse to attend such a ceremony if you are invited. If you don't live in the South, nice virtue signaling...Yankee.