It is not necessarily true that you can't use tile over a wood framed, pier and beam floor.
You might ask several other contractors for their opinions.
You might be able to add an underlayment which stabilizes the floor enough to allow tile.

One solution over here canbe steel plates (free translation would be dovetail plates) and lightweight concrete to make sturdy floors on top of beams. Not sure how the rest of the planet does this (I'm not familiar with other constructions).

In our home (also beams) we installed a floating 20mm plywood floor, vapor retardant plastic foil on top, about 6cm reinforced lightweight concrete with water heating in it, then grading it towards the drain with mortar, a full closed layer of tile glue and after that tiles on top. This is heavier than the solution posted above, so we did have the beams checked, if they could handle the load. (if not, have extra beams installed, this will also prevent deflection)

Note: A concrete floor will make a thicker floor, thus requires higher doorsills, etc.
There are great quality pvc floors available, use them in combination with a shower pan.
There are properties to consider when picking a bathroom floor material.
Make a list of pros and cons and see what you 'could live with'.