You can see the relevant joke in the speech below (although amusingly Pitt also called out his Once Upon A Time in Hollywood co-star Leonardo DiCaprio as well).
It was a little awkward, but it basically made a point about the tabloids constantly connecting him with different women in Hollywood.
Things have been so wild that earlier this year Brad Pitt even joked he couldn’t bring his mom to the 2020 Golden Globes because then people would say he was dating her.