-Tail starts below. Don’t really like this one because it looks really stupid when nothing is going on
-Lightsaber it. A reasonable compromise for all parties
-Retractable fish/reptile-looking thingy— this one is good. Optimal choice
-Retractable human thingy— NEVER, NEVER DO THIS
-They transform into a human being with legs and whatnot— what are you even writing about mermen for? Get out of my sight
-The UPPER half is the fish part— I’m already on my way to kill you
As someone who has legitimately thought about this from a specbio perspective: the best answer imo is cartilaginous fishfolk (so, bottom half shark, skate, or ray. Or go fantastical and modify a more traditional-looking fish half.) Male cartilaginous fish have a pair of modified pelvic fins, called claspers, that act as transport pumps for sperm.
Arguments for merperson claspers include:
-Capacity for human-like penetrative sex, as claspers are used for internal fertilization.
-While claspers don't have human-like erections, they can be raised and lowered for copulation.
-Two peens. In real life, only one clasper is used at a time, but you can get creative.
-In terms of art and mental visualization, it is subtle and undistracting. The average person can't even identify a male shark despite them having their dicks out 24/7.
-The claspers in many species open up to hold themselves in place, if you're into that sort of thing. I've yet to find effective diagrams but I assume it's similar to knotting in a/b/o or werewolf content.
-Some species' claspers have ridges, spines, or hooks if you're real freaky.
-Chondrichthyan ejaculation is wild. The fish's body siphons in water from outside, then pumps it out along the claspers to deliver semen. This results in more fluid delivered at more force than typical human ejaculation.
-Two peens.