One Fish, Two Fish, Half Fish, Marine Biologist.

H2O no.

~CW: Still Teratophilia Aka Monster fucking so skip this whole chapter if you don't wanna read that.~

Izuku was straddling the blonde's lap as a three of their cocks met. The smaller squeaked when he felt Katsuki wrap around his and start jerking him off. They languidly stroked the shaft of his cock making him blush brightly. Katsuki tilted his head up so they could kiss again and Izuku hummed into it. It felt good for both of them. Though Katsuki knew this isn't how his species usually does it he wants to please Izuku before they officially mate. He doesn't want Izuku to think they'll have boring unpleasant sex for they rest of their lives.

So he manipulated his cocks like they were extra hands and had them stroke Izuku smaller dick. He seemed to like it so Katsuki knew he was doing something right. Izuku gasped when he felt the soft but firm dicks stroke his own. This was definitely weird but Izuku didn't care because he wanted more. All of Katsuki so he pulled back from the kiss to look at the blonde. He pecked his lips when Katsuki looked confused but then he spoke.

"Kacchan we can move on to other things." Izuku said shyly

"Eager are we?" The blonde asked with a cocky grin when he looked at the other

Izuku didn't deny anything though as he nodded his head.
"Yes, I want more of Kacchan." He stated proudly

"I want more of you too." Katsuki replied as his cocks stroked Izuku one last time before letting go

"Can you turn around for me?" He asked kindly

"Of course." Izuku said as he got up a little embarrassed but did what Katsuki asked

He leaned against the merman's chest and felt scaled arms wrap around his waist to hold him in place. Then the blonde leaned back a bit leaning on the wall of the pool to get into position. Izuku was flushed but he watched Katsuki's tail slowly move up and down to keep them from sinking to the bottom then he felt it. The tapered head of one of Katsuki cocks poke at his hole making him gasp and his own dick throb. He knew his cock was already leaking precum even though his lower half was under water.

"Relax baby." Katsuki said making a shiver run up Izuku spine when he place a gentle kiss to his shoulder

"Y- yes Kacchan." He nodded then the cock slowly entered him getting wider the further it went in

Katsuki tuned Izuku's head towards him and they kissed when the blonde's groin met the smaller's ass. His other cock fondle Izuku small balls before trying to slid in with his other dick. The biologist's body lit up when not cocks sat deep inside him, making him feel so full and when they started to move thrusting in and out without Katsuki moving his hips, it was jaw dropping. The two stopped kissing as Izuku looked toward his cock as if trying to see what was happening to his ass. His mouth was slightly parted and he couldn't speak because the pleasure silenced him. Katsuki worried for a moment when the smaller went completely quiet while looking down at himself in the blue tinted water of the pool they were in.

"De- Izuku?" The fishman asked them flinched when Izuku moaned out a lewd whine

He worried for a moment before Izuku said don't stop. Then he realized the sound was a good one, a sexy one, because the more his cocks alternated their thrusting inside his hole. Things were starting to get slimy inside from Katsuki's precum, but Izuku didn't care. Not when his head was fuzzy with pleasure and his body felt like it was floating. Technically it sort of was since they were in the water, but whatever. The kissing, the touching, the thrusting, it was all almost too much for Izuku yet he wanted more. He greedy which was a surprise to him but was quick dismissed in favor of kissing Katsuki harder. His cocks were so deep inside him, filling him up, and stretching his walls. He can't wait to feel Katsuki cum in him. He's sure it'll feel just as good because how could it not.

Eventually he got his wish after the cocks starting thrusting in unison. Katsuki's face was flushed he had never felt this good before. Izuku's insides were like nothing he's ever experienced and maybe that made him cum a little prematurely. He hoped Izuku didn't care or would even realize it. His slimy dicks sank deep inside, as far as they could go, and planted his warm creamy seed inside the smaller's ass that felt ethereal. Every part of Izuku was too good to exist, to dangerous to be left alive, but Katsuki had him all to himself and that was the best part.