Why the hole at back ?

Saamee thoughtss (￣∇￣")

Maybe cuz they're man i wonder

No, if u have read a merman bl or girl before it had been always at the form because mermaid is intersex (๑•ㅂ•)و✧, or maybe the author just want to put hole in th back

Sorry, for more details

Its cause mermaid was given a intersex depiction.
So they have both a (penis and vagina)

But not all men has a penis in mermaid form so maybe thats why

Nah, it's in the front because it's in the front for fishes. Fishes have their spine there. It's like having a hole on your back along your spine. It's weird and make zero anatomical sense.

Yeah

Wow that actly make sense (≧∀≦)