The amount of time Briseis and Achilles's relationship spends on screen is exactly nil. All we have to believe Achilles when he says he loves her is his snit when she was taken, which is just as likely to be wounded pride as anything else.
Conversely, we actually see Achilles and Patroclus interacting and showing affection and caring for one other. So ask Briseis if you like, but I doubt she'd have anything worthwhile to say.