So many people assign "rules" to the romance genre that they don't to other genres. It seems to go hand in hand with the favourite assumption that any bedroom scene in a romance is a factual report of the author's own experience.
As others pointed out above, nobody calls out writers in other genres for having no experience with murdering people/fighting aliens/slaying dragons... Whether a person is in a relationship or not, they see relationships in real life and in the media every day. I detest the assumption that romance is just about a couple of people having sex. It's first and foremost about strong characterisation, and any good writer can do that.
I have a romance set in the 1880s coming out in September. I've never lived in the Victorian era, but I did my research.
"Write what you know" is a term I half agree with and half disagree with. For example, I'm a former ballet dancer, and almost every book I've read with a character who is a dancer has been a head-desk read for me. On the other hand, I read a fantastic series set in Australia, by an author who had never been here, but who had taken her research seriously. (Just please never make the mistake of using
g'day as an alternative to
goodbye; it means
hello!)
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You can write anything that makes you happy or that you feel the need to express. The rub comes when you want to publish what you've written. Make sure you've read a ton of romances if you are interested in pursuing publication for what you write, because there are definite genre and sub-genre reader expectations.
On the other hand, that ^^^^!!