The short answer is: If you’re writing a romance, you need a Happily-Ever-After (HEA).

The slightly longer answer is: In the romance genre the story should end in an HEA, in some cases a HFN (Happy For Now) will do, and exceptions might be made in a series as long as that ends in an HEA.

The detailed answer is:

The most basic definition of a romance is that at the end of the book the couple is happily together.
This is the one rule you cannot break, but—depending on circumstances—you might bend it.

With all writing “rules” you have to take a look where they come from, and what they aim at: In this case, it’s all about genre expectations. 

Marketing your book as part of a genre means that you make a promise to the reader that certain elements will be in that book. Compare it to a label on a glass of jam: if the label says strawberry, you wouldn’t want to find raspberry jam inside.

Genre means that you label your book so the reader can pick more easily what they want to read. It’s not about pressuring you into writing something you don’t want to; it’s a marketing tool that helps you reach the right readers.

For readers to be happy, in a murder mystery there has to be, well, a murder—and someone who investigates it. (No one would ever question that.) 
In a romance it’s all about the HEA. A romance reader isn’t reading a book to find out IF the main characters will be together in the end, they want to read HOW they get together.

But what if I write a series following the same couple?
 

Here’s where you actually need to bend the rules: by stretching out the plot points. 
Most successful romances follow a certain structure of plot points—the so-called romance beats.
We will look closer at those in another Library entry, but overall there are four phases:
- Setup (characters getting to know each other)

- Falling in love (characters falling in love)

- Retreating from love (characters' conflict gets in the way)
- Fighting for love (characters changing their ways to be together)

When you’re writing a series, you need to stretch out those romance beats over the course of the series, but be careful when picking the beats to end each book on.

Let’s compare:

In a stand-alone book, the couple might fall in love around the midpoint, retreat again a bit later, and after overcoming their conflicts, the two finally find their HEA at the end of the book.

How you could spread the phases over the course of a trilogy: 

- You could end the first volume with the couple getting together (falling in love),

- they might break up in the second book but end with the realization that they want

 their love interest back,

-and in the third book fight for love until they get their HEA.

- Another more slow burn approach might be that they only start to fall for each other in the first

 book, ending with a “promise of love” (to the reader, not necessarily the characters);

- in the second book they finally fall for each other, ending in a HFN,

- and the third book might start with the retreat and end with fighting for love and the HEA. 

- Or they fall hard for each other quickly and end in what seems to be a HEA in the first book,

- but their histories or secrets create a conflict that keeps them apart in the second book,

- and they overcome that to finally be together in the third.

Keep in mind that it can be helpful to let readers know in your marketing (e.g. the blurb), that the series will have an HEA and also warn about potential cliffhangers.

I have picked trilogies for those examples, but it would be similar with longer series. Ideally, you create a (coarse) concept for the series, to know which elements you have to place and on which beat each book will end. If you keep it very simple, this approach works as well for pantsers as it does for plotters.

It’s often enough to decide that the second book needs to end on e.g. the couple breaking up. How they get there and why this happens you can still discover during writing if this is how you work best. 
 

The longer a series gets, which easily can happen with fantasy romance, the more difficult will it be though to stretch the usual romance beats.

If you’re not careful, it can end in what feels like a never-ending back and forth between the couple that isn’t very satisfying for the reader. The other way is a very slow burn with the romance arc stretched thin throughout the books, and more weight on the fantasy adventure.

This is not unusual, but here you’d need to have an eye on genre again – and maybe move away from “romance” to “romantic fantasy”, which implies that the romance arc isn’t the main focus. It’s all about setting expectations right, so readers will know what awaits them.

I don’t want my story to end with an HEA. What’s with “write what you want”?

I see “write what you want” as misleading advice, because it’s very inconsiderate. It doesn’t regard what you want to achieve and therefore in most cases is unhelpful, unless you truly don’t care about any readership and only write for yourself. (You might be so fortunate to get everything right on instinct alone, but the truth is that most of us have to practice).

But: of course you can write a story without an HEA!

It doesn’t mean that anything is wrong with your book—it only means that the romance genre is not the right place for it. Instead find the genre with your targeted readership.

Possible options could be broader fantasy or sci-fi categories, or, especially if your book is historical or contemporary, you could market it as love story, or women’s fiction.

It’s all about finding the right readers for your story. Maybe your main characters do not get an HEA, but you and your book just might.