These specific monomers can be found in nature, and they are widely used in industrial applications beyond making plastics, for example they can fuel welding torches.
The monomer of PE is the gas ethylene, which is a plant hormone among other things, and if you combine it with hydrogen from water it makes ethanol, which is the kind of alcohol that is only mildly poisonous so we drink it. It’s the easiest way to get alcohol without fermentation, so we use it in industry. So I guess mostly it is.
Propylene, which is the monomer of polypropylene, is a gas that’s toxic in large concentrations, apparently it’s what makes forest fires poisonous, but it’s fine to have a little bit in the air, and it’s at least not carcinogenic.
The problem with plastics is not that they are toxic, it’s that we make too much and litter in everywhere, including our own body.