Jellyfish have male and female individuals just like humans and most other living things.
The males release long strands of sperm that float in the water and are picked up by the female jellyfish. There is some very good evidence that the sperm is highly attracted to certain chemicals in the female egg, thereby increasing the chance of fertilization. See the handout mentioned above for more information of the life cycle of Aurelia and other jellies. But this amount of poison varies drastically. They also don't have any major predators so they don't have to defend themselves.