limestone requires a very ridgid deflection , must be set w proper mortars you Cannot seal limestone with just any sealer. You want to use an impregnator solvent based sealer that will allow the stone to beeathe, we call these sealers Vapor permeable.

Id listen to your builder he is telling you he iis not a qualified tilile setter or he doesnt have one. He may also want to steer you to an easier cheaper material for the floors deflection or budget ...

You should know Limestone is a very fickle material, long term maintanance potential calcification issues may occur, i could talk you out of it differently than your builder is in about 5 mins flat!

Marble is metamophisized limestone and is harder and more durable than limestone. Read all the horror threads about marble. Then know that limestone is softer, not as dense, and the darker colors etch white. All bathroom cleaners are acid based, and will etch marble and limestone. Urine etches it. Lots of skin care products are acid based and will etch it. Then there are the scratches, chips, stun marks, and stains.

If you can’t live with that reality, then you need to be only looking at porcelain.