I was sure it'd be impossible to find a more horrifying mating ritual than the praying mantis, but i think i have found it. You can thank the salmon. You've probably heard the old tale of how salmon swim upstream to mate, but that's not painting the true, terrifying picture....
Let's say you and a bunch of friends make a journey to your childhood hometown for an orgy (look, you need to accept that the animal kingdom just has looser rules here). But the trip is dangerous--you'll have to walk across a wilderness filled with enemies, and the air at the destination will kill you if you breathe too much of it. So it's actually kind of like the fellowship in Lord of the Rings traveling to Mordor, only instead of saving Middle Earth, you're looking to grab lots o' strange titty.
By the time you make it to the destination, the trip has been so hard on you and your companions that everyone is wounded or literally rotting on their feet. You've been reduced to a whole bunch of horny zombies. Yup that's right, zombies... This mass of horny zombies congregates in a disgusting, debauched tangle of genitals and rotting flesh. And as soon as everyone is done sexing it up, the entire undead swinger's convention just drops dead.
Why in the hell do salmon go through this? Well, they're born in fresh water, but migrate to the sea as they mature. However, at some point in their adult life, a primal desire drives them to swim back to the freshwater they were born in to mate and lay their eggs. Even more amazing, the drive is synchronized in every generation, so everybody you grew up with as a salmon just starts leaving around the same time.
In order to survive the journey and deal with the change from salt to freshwater, a lot of hormonal changes take place. Unfortunately, these changes practically destroy their bodies, turning them into zombie fish. What with all the swimming through treacherous, brackish water, surrounded by predators like bears, all while your own body starts shutting down...it's a wonder any of them even make it at all.
And once it's done, they all just die, like some kind of underwater Jamestown. The next generation will benefit by the increase in nutrients in the stream caused by a whole lot of parents-turned-fertilizer, ensuring that the next generation can live on and...die horribly as well?
Could it possibly get any worse? Oh yeah, it can: sex for salmon is external and without anything we humans would regard as pleasure, which is why you should never ever date a mermaid.