Polystyrene IS a known endocrine disruptor. Now, is there PROOF that it will definitely harm your frogs, or affect breeding, sex ratio's, lifespan, muscle formation, bone formation, bone density, etc.
No. There is no proof. There is no institution spending hundreds of thousands of dollars to find the long term effects of exposure to polystyrene. This would take years of study over many, multiple pairs of breeding frogs.
Straight up, this research will most likely never be done.
Therefor, it is up to each of us to decide what the dangers may be. Now polystyrene is a know endocrine disruptor. This can cause cancer, death, affect muscle and bone formation, influence sex ratios, change hormone levels, affect breeding, and block egg formation (birth control pills are CONTROLLED endocrine disruptors). There is really no way of knowing what it may be doing to your frogs that you cannot see. This would be a totally uncontrolled exposure.
Now in dryer viv conditions, this may not be a problem. Our vivs, however, are very moist and humid. This makes conditions ripe for leaching chemicals.
What it all boils down to is this. Styrofoam/polystyrene MAY cause problems. It has been proven to have the potential to be a problem. There are safer choices available. Personally, I will always choose the safer choices. There is NO polystyrene in any of my builds and there will never be.