Slaves marrying from other plantations, how did this work?

I'm from Australia, so I have had little education on the slave trade in America and these people's day-to-day lives. So excuse my ignorance on this topic.

But I'm reading 'George Washington - A Life' by Ron Chernow. And he mentions there being no case documented in which Washington denied slaves the right to marry between plantations.

This had me confused as I didn't think they would ever really meet or see eachother between plantations. In saying that as I mentioned I don't know the schedule that slaves adhered to and the opportunity they would have for meeting other people they wished to marry.