Are monarch butterflies poisonous

Monarchs are poisonous because they eat plants that are themselves poisonous.

There's a distinction to be made between "poisonous" and "venomous". Monarchs do not produce any venom, nor do they sting or bite (they're actually not even capable of "biting" as adults; they live solely on liquids) you have to eat them for the poisons to get into your system.
Some animals do not eat Monarch Butterflies (caterpillar, chrysalises, or adults) because the poisons in their bodies taste terrible to them. Other animals do eat them. They are not poisonous to touch. They do have poisons in their bodies that are from their host plant, milkweed.