I'm writing a story about a merman, and thought I'd try to figure out some kinky little habit a merman might have in mating that's different from your average male. So I turned to google and wow, there are some very strange rituals/habits.
1. Anglerfish - the male is born with extremely well developed olfactory organs (it means he can sniff out a female) They have no stomach or digestive system and are dependent upon finding a female in order to survive. They sniff out the pheromones of a female and then bites into her flank and releases an enzyme that digests the skin of his mouth and her body and they end up fused down to the blood vessel level. (eeeww) . Basically all that is left of the male is his gonads which release sperm into the female.
2. HoneyBee and Male Wasp Spider: The queen of the colony will select a drone to mate with - but that drone's genitals will actually explode and snap off inside her, plugging her, preventing other drones from fertilizing her. So the old expression about a guy dying to have sex -- yup!
3. Bonobo (or Pygmy Chimpanzee) uses sex as a day-to-day method of communication. According to Wikipedia, and a few other sources, sex is used as a greeting, for conflict resolution and post-conflict reconciliation. Females will trade favours in exchange for food. And they will engage in face-to-face genital sex (rare amongst animals), including female-female, male-female and male-male), tongue kissing and oral sex. (male-male sex is sometimes referred to as penis fencing!)
4. the Argonaut (a type of octopus) has a detachable penis. If the male sees a female it likes, it detaches its penis and the penis swims over to the female all by itself.
5. The Argentine Lake Duck has the world's longest penis of all vertebrates in relation to his body length. The penis coils up but when extended, can equal the length of the duck - which is up to 17 inches long.
6. Porcupines: Remember the old joke about how Porcupines do it? Veeerrry Carefully? It's a bit more gross than that. The male will spray the female from top to bottom with a huge quantity of his urine - they can spray from up to 7 feet away. The female can accept the urine, or shake it off, to beat the male up. And this can go on for weeks. The actual act? The spines and quills of the female relax and go flat and the male enters from behind. The female apparently doesn't like it if the male tires and will bug him to continue (does this sound familiar?) If he's ... um, tuckered out, she'll find another male to mate with.
7. Gorilla males have a penis only 1 1/2 inches long - but he's usually surrounded by 30 - 50 females and no males. So the females can't complain too loudy. But if you want to insult a guy, this would be a great one that I'll bet not many guys would get.
9. Hyena females have labia that resemble a scrotum, and testes. And their clitoris is so long it rivals the size of a males penis and can be fully erectile. During mating, the 'pseudopenis' retracts like a "shirt sleeve being pushed up and during birth it stretches so much it looks like a water balloon" [source]
10. Giraffes: The male will take a mouthful of the females urine and taste it. If it tastes good to him then he'll begin courting her. Eeewww!
11. The dolphin has a retractable penis. And it's prehensile. Which means it can behave like a finger and swivel around. They can mate often, and do, but they don't last very long - in fact only 12 seconds. Typical male!
12. Geese and Gulls often exhibit homosexual behaviour. Two male geese may form a bond, a female may insert herself in their mating behaviour and be accepted by them and the three of them may form a family, while up to 15% of Santa Barbara Seagulls exhibit lesbian behaviour, even to the point of laying sterile eggs.
13. I saved the best for last: the turtle - the biggest neck wins. The male will bob his head and bellow, then when he's attracted a female, he'll snip at her until she withdraws her legs (which means she can't escape him) And then mating can apparently last for hours. (What do you expect, he's a turtle, he's gonna move slow) Frustrated males who can't find a mate have been observed humping rocks, LOL.