Romance novels don't all agree on the rules, and the rules change over time.
Often, the rules in any particular novel are left implicit in the text and can be deduced from the characters' comments and actions.
Occasionally, however, the rules are discussed openly, as in Nora Roberts' Bay of Sighs (2016), and such scenes draw attention to the fact that the rules (like all the other aspects of romance which can be classified as political, or as "social issues") are present in romances all the time.
In Roberts' novel the rules have to be discussed explicitly because one of the characters, Annika, is a mermaid, and therefore comes from a very different culture. She needs to have human social behaviour explained to her because she is in love with Sawyer, a human, and wants to have a romantic relationship with him. She knows the rules which bind mermaids:
"[...] I'm not permitted to kiss a land person the first time. He must want me, show me. He must choose."
"Why is that?"
"Our females have the power to lure men - humans. To seduce so the choice isn't a choice for them. Long ago, and not so long ago, some of my kind lured men, sailors and explorers."
"Yes. The song of the siren is beautiful and powerful, but it can be dangerous to the human she calls." (93)
However, she doesn't know all the rules governing human behaviour. After Sawyer has kissed her she wants to know why he doesn't
"[...] ask for sex. I don't know if I'm allowed to ask for sex. I don't know the rules of this."
[...] He jumped on that concept. "There are rules. Lots of complicated rules." (146)
Annika's other companions then attempt to explain the rules to her, and some of them have more complicated versions than others:
"Complicated." Riley snorted. "I say simple. My top three? Both parties willing, available, and clean." [...]
"Riley." Sasha rolled her eyes. "Different rules for different people. Or not rules so much as ... sensibillities, and it's not always easy to explain."
Riley ticked off on her fingers. "Willing, available, clean."
"An important foundation," Sasha agreed. (147)
Riley then asks a bit more about the rules for merpeople:
"Are there gay merpeople?" [...] Can you mate with someone of the same sex?"
"Of course - differently because of the body, and there will be no young created, but you want who you want, yes? Love who you love?
"Cheers to that." [...]
"Is one of your rules you cannot?"
"We're eliminating that rule. Slower in some places, but we're working on it."
Annika huffed out a breath, frowned at her drink. "Are all the rules stupid?"
"Maybe some are, and the rules depend." (154)
Since there can be serious consequences for those who break the rules, and since some rules can be deeply harmful to certain individuals, it's important to make sure that the rules are not "stupid." However, those imbedded within a particular set of rules can't always see which of them are "stupid" and which aren't. Romance novels can raise questions about the rules, and the communities of romance readers are a place where discussions about the rules can and do take place.
Roberts, Nora. Bay of Sighs. London: Piatkus, 2016.
The image is of John Reinhard Weguelin's Mermaid (1906). It's in the public domain and I found it at Wikimedia Commons.