Image caption Leah (left) and Bea Koch says that romantic fiction is wrongly looked down on

Romance novels are a billion dollar industry, vastly outselling science fiction, mystery and literary books.

And there's only one rule for writing a romance - it has to have a happy ending.

Yet the romance genre has long been dismissed as smut or trashy by many in, and out, of the publishing world - a fact that mystifies sisters Bea and Leah Koch, who last month opened the US's first exclusively romantic fiction bookstore.

Their shop in Los Angeles is called The Ripped Bodice, and the store's motto is "smart girls read romance".

"The fact that up until now the best-selling genre in America didn't have its own bookstore honestly didn't seem fair to us," says Leah Koch.

"You have comic book stores, and science fiction bookstores, and mystery bookstores - probably all of which combined romance sells more than. We deserve our own book store, too."

'Unique offering'

Primarily read by women, romance is by far the best-selling genre of fiction in North America - with $1.08bn (£760m) in sales in 2013, according to the Romance Writers of America.

And readers of romantic fiction "want a knowledgeable bookseller who's not going to laugh at them for reading romance," says Bea, a Renaissance scholar with degrees from Yale and New York University.

Media playback is unsupported on your device Media caption "You are here to have a good time, to find something that might make you smile"

Yet Bea and Leah, who grew up in Chicago, say that the romance sections at most bookshops are woefully inadequate - and you can forget about finding staff who can help with recommendations.

"The stigma against romance is extremely sexist," says Leah, 23.

"It is really interesting when you look at teenage girls... there is just an inherent dismissal of whatever teenage girls like, despite the fact that they have huge buying power, and should not be dismissed. And I do think that [sexism] extends to women of all ages."

The Ripped Bodice is a clean, well-lit place, devoted to the many sub-genres of romance, such as cowboys, aliens, Vikings, biker dudes and the paranormal.

There's also a large erotic section, a Spanish-language area and plenty for young adults, as well as the lesbian, gay, bisexual, trans and queer communities.

In the historical section, books include Jane Austen spinoffs and romantic tales among tartan-clad Scottish highlanders.

"Love can come in many forms," says Bea, 26, with a smile.

Image copyright Getty Images Image caption The success of Fifty Shades of Grey has given romantic fiction a significant boost

The store was opened with the help of romance fans.

As young, untested entrepreneurs, the sisters didn't expect banks to be much help, so they went directly to the internet to raise seed money from their peers.

They used the online crowd-funding platform Kickstarter to raise nearly $100,000 to open the shop.

They say that business has exceeded their expectations over their first few weeks, and that their very specialism will be the key to their store's long-term success.

Leah says: "We think niche book stores are the wave of the future, especially from a business perspective and for making enough money.

"To compete with Amazon you have to offer something unique."

Image caption The Ripped Bodice is already proving popular with customers

The sisters say that romance readers want a place to gather and talk about the books, and meet authors. A place where fans of the genre can read romance without shame.

And book industry figures show that buyers of romantic fiction are more loyal to brick and mortar stores than general readers.

While only 41% of general book sales in the US are now from physical shops, last year's Nielsen's Romance Book Buyer report found that 56% of romance fans still primarily buy from brick and mortar stores.

Image caption The sisters used the online crowd-funding platform Kickstarter to raise nearly $100,000 to open their shop

Romance fans also read voraciously. According to the Nielsen report, some 6% of buyers purchase romance books more than once a week, and 15% do so at least once a week.

And the stigma against romance is changing. The success of the novel 50 Shades of Grey stunned the publishing world, and introduced a lot of new readers - including more men - to the genre.

Romance is, however, still primarily read and created by women - who make up more than 85% of romance readers.

'Good time'

Best-selling US romance novelist Eloisa James (real name Mary Bly) says it is "a wonderful idea" to have a bookshop that only sells romantic fiction.

She created a pen name for writing romance because she didn't want it to jeopardise her academic career - she is today professor of literature at Fordham University in New York.

Image copyright ALEXANDER JOE Image caption Romantic fiction is big business

Ms James was born out of economic necessity back in the late 1990s when Prof Bly needed to pay off her student debt following studies at Harvard, Oxford and Yale.

"So I turned to a genre which I've always loved," she says, adding that the advance on her first book paid off her student loans.

Ms James has since written 25 books, 21 of them making it onto the New York Times bestseller list.

When Eloisa James has a party, or reading and signing, at a bookstore, she says the storeowners are often shocked by the size of the crowds that show up, especially in bookstores with little or no romance section.

She adds that general bookshops often say they don't have the audience for romance, a claim she says is "ridiculous".

Image caption Jane Austen's novels still do handsome business

Back at The Ripped Bodice, the Koch sisters intend to make the shop a profitable enterprise. They have big plans for book clubs and community events.

On a daily basis, Bea looks after the historical books, while Leah, who went to the University of Southern California, looks after the contemporary releases.

"We always tell people that they should expect The Ripped Bodice to be a serious business that doesn't take itself too seriously," Leah says.

"This is our livelihood. It's our business, we take it very seriously - however we are selling romance novels."

Bea chimes in: "You are here to have a good time. To find something that might make you smile.

"That's what we want to be - a respite from everything else that's happening in the world."