Probably a mix of power/control for me tbh, though a no is a no if it ever got to that point. And as a previous poster has pointed out, it is quite fun when FS is offered to you and not to others (apparently)
Tbh I've stayed away from brothels since a lot of girls know exactly what you're there for and end up being robotic anyway. I've found more joy fs in a massage shop where there's less pressure...