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

Reproductive.

Katsuki gestured to the two little mounds of scales. "This is my genital slit." He said

Izuku frowned trying not to stare too long. He kind of wanted to run his finger down the so called slit.
"But how-" The smaller started as he looked at the blonde's face who interrupted him

"My dicks are inside." Katsuki told the other

Oh gosh. Dicks. As in more than one. As in the blonde has two or more. Izuku blushed brightly, but tried to remain calm and not think to much about it.
"Y- you have more than one?" He asked looking down at his notebook

Katsuki frowned confused.
"Yeah, don't you?" He questioned

Izuku shook his head.
"No. Humans just have one pe- penis." He said avoiding the other's eyes

Katsuki scrunched up his nose.
"Huh. Weird. Why's yours like that?" He shook his head

"It's just how our anatomy is." Izuku shrugged

"Um anyway please continue." He told the blonde wanting to switch the subject back to him

"Right uh when it's time to mate then it comes out until the mating process is done then it retracts back in."  Katsuki explained

Izuku started writing things down.
"Oh that's probably for protection right?" He asked

"Yeah." Katsuki nodded

"So then your testicles are inside too?" Izuku looked over at the taller

"Aren't everyones?" Katsuki questioned

Izuku shook his head.
"Um no Kacchan that's different too. Human's well biological male's genitals are external. Both the penis and the testicles."

Katsuki pressed a finger to Midoriya's shorts making the other tense.
"Is that why you wear clothes? To cover your dick?" He asked

Izuku nodded with tinted pink ears.
"We'll mostly. It's also for protection since we don't have fur or some sort of armor." He said

Katsuki moved his finger and looked out over the water.
"Mm. Strange." He

"Probably to you." Izuku laughed lightly trying not to feel awkward

"Whatever." Katsuki rolled his eyes

Izuku cleared his throat.
"Um you mentioned ma- mating. Have you ma- ted with anyone?" He asked nervously

Katsuki looked at the other with an unamused expression.
"Deku I've been here since I was fifteen, you think I've mated with anyone?" He narrowed his red eyes

Izuku looked to the side.
"Oh right..." He trailed off

"But do you ever want to? I mean do you want a family of your own?" The marine biologist asked

"Do you?" Katsuki turned the question back to him

Izuku smiled softly and nodded.
"I'd like to one day, but I'd have to adopt or get a surrogate since I can't have children." He said

Katsuki the frowned.
"I thought the females of your species could bare children."

Izuku nodded again.
"They can but I uh don't exactly like women." He said rubbing the back of his neck

Katsuki frowned harder.
"You a misogynist? Cause that's shitty." He said crossing his warms

Izuku's eyes went wide as he shook his hands and head.
"What no! Never!" He exclaimed

"Then what the hell are you talking about?" Katsuki asked

Izuku blushed brighter than he had before. His whole face turned red.
"I- I like men Kacchan. Well mostly. I'm bisexual so I like both, but I lean towards men."