Do you prefer euphemisms or technical terms to refer to genitalia in your writing? Why?

Trace the etymology of even the technical terms back far enough and they'll have euphemistic origins. To use an easy example, Merriam-Webster.com says this about the etymology of the word vagina [www.merriam-webster.com]:

borrowed from New Latin, going back to Latin vāgīna "scabbard, sheath," of uncertain origin

We're privileging a certain subset of words by saying "these are the professional / formal / official ones" and "these are the slang terms." They're all slang terms. We've just granted some the imprimatur of legitimacy.

At the same time, our stories exist in a context. Some of that context, the story provides to the readers. Some of that context comes from the readers themselves. If a character (or the narrator) is using technical terms, that can tell the reader things.

Given the context of the story and the audience, what's the right word at this moment in the story to convey what we need to convey?

In Trans Wizard Harriet Porber and the Bad Boy Parasaurolophus, Chuck Tingle [www.chucktingle.com]'s protagonist, Harriet Porber, and her love interest, Snabe, are both transgender characters. In the first sex scene between the characters, we get this exchange:

"Before we go any farther, this is my cock" the dinosaur informs me. "Don't call it a strap-on, don't even think of it as a strap-on. This is my dick. Understood?"
(...)
"You can touch it," I explain, "and you can suck it, but call it what it is: not a dick. That's my clit."

In real life, probably these characters wouldn't need to say all of that but they're not saying all of that for each other. They're saying all of that for us, the readers. They're using that language to tell us that they are using terms in a different way than we may be used to.

Even if you're not including sexual content, there's something for those of us on the speculative end of things to consider. Are we introducing our in-universe terms in an accessible way?

In a more direct answer to the question: I like a variety of words used at the right moments.

Do you include bodily functions in your stories (do your characters poo)?

One of the earliest pieces of feedback I ever got about my writing was "your characters eat in your stories." Being autistic, I'm kind of used to this kind of "feedback." I have to figure out what it means. It's a thing but I have actually inferred some of the most important advice of my life from this kind of feedback. In this case, what the person meant was that most fiction doesn't include meal scenes just because it was time for the characters to eat. They didn't see what having the characters eating contributed to the story.

There's a relatively early scene in Firefly where we see Mal push a fold out urinal back into the wall and zip his pants. The purpose behind that scene was to demonstrate an attempt to convey a realism in the universe of Firefly that wasn't present in Star Trek at the time. It's a blink and you'll miss it moment, but it's there and it serves a purpose.

One of the upsides of erotica is that you can include in your story anything you find arousing. You might even be able to introduce your readers to something they didn't think they were interested in. Due to the limitations in certain markets, I don't have any plans to include scatological or urological content in any of my upcoming stories. But I totally would if I could reasonably publish it and the story was one of those.