The problem with titanium alloys is unless finely polished/ burnished they have a nasty tendency to behave in an abrasive way when something slides over the interface. The bottom bracket in a frame are likely tapped, and screwing the cup in dry will likely cause instant gauling and microwelding. The parts won't get a chance to be damaged from corrosion or eventual solid state diffusion.
Grease or antiseize are absolutely necessary.