Hypothetical (or so I would hope):
If you HAD to choose one type of mermaid/merman to mate with, which one would you choose? Nevermind the logistics of Tab A going into Slot B, but just answer the question based on your first instinct.
Yes, this is a Futurama reference. 
It’s a not a mermaid if it’s a fish on top.
Aren’t women mermaids already?
Holly: Somehow I’d imagined she’d be a woman on the top and a fish on the bottom.
Cat: No, that’s the stupid way around!
The sex may not be as…frictionfull, but I can’t stand those dead fish eyes staring at me. Bad enough having sex with your partially completed dinner only feet away.
“Why couldn’t she be the other kind of mermaid, with the fish part on the top and the lady part on the bottom!”
Fishy part on the bottom. Though fishy part on the top would look funnier.
Fishy bottom, human top…I want to hear some hot talk during sex, not bubble noises.
Hey, at least where not talking about sex with a guy named Seth Brundle.
Smeg! I came in here to make that quote…
Engage JealousPoutyMode…
I defer any cool-points for my above Red Dwarf reference to the obviously more sci-fi-minded MacTech who hails from the unfashionable sector ZZ9 plural Z Alpha. 
Thread title makes me laugh every time I see it so, thanks.
sniggers fishy bottom
Chopper9760:
Gorsnak: I dunno, I can’t make heads or tails out of that song.
Gorsnak beat me to it.
Aww, but I’m just 'zis guy, you know? 
I voted for human top half. I would easily be able to interact a normal mermaid as a person, even though she has a tail. A creature with a fish brain, fish face, and no hands would be completely alien.
Looks like I’m in the minority.
We’re talking about mating here. You don’t have to take them out to dinner or marry them. You just have to mate with them, once hopefully.
Choose fishy top and go doggy style. You can imagine they’re just wearing the top half of a fancy dress costume.
Yeah, but I think the strange rasping sound being made through your date’s gills might end up being a real turn-off.
Fishy bottom all the way.
The things you could do with a large mouthed bass…
Yeah, but she’d be a real cheap date. Just sprinkle a little food on her ceiling.
OK, this made me LOL.
Ah, I was thining the same as you until I noticed in the OP:
That tells me that mating would be OK no matter the configuration. I think even doggy style it’d be too much of a turnoff to get Mr. Happy to work enough to necessitate mating.