Now, I don't know erotica awesomely well yet, but I do know how to research things. I have written many, many technical papers in college, and I think those techniques I used can be used and adapted for this. Let's see, shall we?
First, we're going to research "cuckolding" or the "cuckold" niche. I'm completely uninterested in this at all, so this isn't like giving away my secrets or anything. I'll learn something, you'll learn something, and hopefully we will all benefit. Back in the day, the internet wasn't something really used for research, but now we have all the information we have at our fingertips.
Now that we've decided what we are researching, what we want to do is figure out what information we need exactly. For tech papers, I would concentrate on one aspect of the subject at all, because I was basically saying, "Hey, this technique works well for this." or "Hey, if you use this on this, you get this. Here are my findings! Here's the info that backs me up!" So, I was interested in specific technical details. For writing, especially writing to evoke emotion, we want to learn what about the kink evokes what emotion. We have to get inside the head of the reader. Why is cuckolding hot? What makes it hot? What sort of people might be interested in it?
We are off to wikipedia! Wikipedia is the best thing to come of the internet, I swear to god. The basic definition, according to wikipedia, is "a husband with an adulterous wife". Ok, so it's a dude whose girl is stepping out on him. Down below, it has "cuckold as a fetish" so we'll look at that next.
Unlike the traditional definition of the term, in fetish usage a cuckold is complicit in his (or her) partner's sexual "infidelity" and takes masochistic sexual pleasure in it. Cuckolds in the fetish sense also need not be male, and need not be married, although this is the most common pattern, and for all couples some level of pair-bonding intimacy or commitment in their relationship is necessary. Among fetishists, the pose of reluctance—the victimization of the cuckold—is a major element of the paraphilia. (Which may be the reason for the use of the term "cuckold," with its connotations of victimization and inadequacy.)
Alright, so it's not just that she's cheating, but the husband is somehow complicit in it. I think this niche could pair with voyeurism very well... think of the husband jacking it while his wife takes on a big black cock (ok, can pair with interracial well too), getting off on seeing his wife fucked. Reading the rest of the article gives us some very basic information about the fetish.
Where to from here? Well, in Google, type "cuckold fetish forum". The first one that comes up looks interesting, so clicking on that link we find ... something not useful. Hm. The second one looks... not useful. Alright, scrolling down a little bit I see something called XXXBBS, which does look promising, and I think here we're going to find what we are looking for. Except I don't. Just pics and movies.
Wow, or they are saying all these people are sick and demented, which isn't cool. From what it sounds like to me, cuckolding is basically asking your significant other to cheat while you watch or otherwise view (ie, through a video). There appears to be some too where the dominant/cheating partner (the "hotwife" if it's female for example) fucks another guy ("the bull", usually younger and buffed), but then demands the cuckold joins in. There's other fantasies where the hotwife is completely submissive and the bull is almost a caricature of unbridled male sexuality, where he just can't help but to fuck the hotwife into submission.
A couple of forum threads where I found some buried nuggets:
So, we have some conflicting, and interesting and complex, data to process. Let's Google "What makes cuckolding hot". The articles and questions I found here were very useful.
I think this gives you a good start on the research here. Just by going through and reading each of these, you'll get a good understand that this fetish is actually quite complex, and can be done for a number of reasons. One of the things mentioned either in these forums or the articles was that "it can't be done right if you don't love them". Part of that makes this fetish pretty interesting, because most fetishes aren't people orientated like that. Many fetishes are object orientated, or don't require the level of emotional attachment a cuckold does. Another article said that instead of physical anguish, like masochism, they get off on mental anguish. "My hot wife was with another man! How can I compete?" Yet others still are turned on by the idea OF competing with the bull, like you're going to fuck that other guy's sperm RIGHT OUT OF HER PUSSY GOD DAMMIT! Yet others find it hot to watch their hot wife get totally reamed by the bull. All of those, or any combination. It sounds like a fascinating study, one which deals with a lot of emotions, and one which deals with jealousy in a sex positive way... instead of getting angry or lashing out when they get jealous (for that's a component too), they use that jealousy to add fuel to their sex drive.
All of what I found was pretty amazing. And it took me 20 minutes to do it.
Now that we've found some avenues for why cuckolds like what they like and do what they do, you need to make sure you've gone the terms right. As someone on the eroticaauthors subreddit pointed out that using the wrong terms could throw someone right out of their fantasy. We've already learned some, "hotwife" "the bull" and so on. Let's see if we can find some more.
Doing a quick search for 'cuckold terms' brings me to a great site. Maybe not so many terms here, but a really good explanation of the whys and hows of cuckolding.
So, there you have it. Obviously, if you should choose this as your niche to write in, you should do a lot more research than my 15-20 minutes, and take the time to read the articles. This will get you started. Just remember that we're looking for why things are erotic here, and then back up that research with the proper terminology. If you possibly can, after you are done with your draft, try to find someone who is into the kink you've chosen and ask them to read it. That would be the ultimate way to find out if you understand their kink or no. Then, of course, you could ask for specific ways to improve... which leads to more research. Time spent on researching things is never wasted.
Side note: An interesting site I found for some fetishes in general is here: http://www.myotherself.com.au/articles-and-resources/
I haven't explored it at all yet, I just bookmarked it for later perusal.
Last, but not least, if you have Kindle Unlimited, borrow cuckolding books and read them. There is no faster way to learn the terms and emotions behind it that to see what other people are doing.