For the people of Aldagar, who live in proximity to Elvie and Meransia, their rules of ascension are considered quite lax.
Same-sex marriages are not frowned upon and are treated as normal, even for the nobility or royalty. The Ruling Two can be m/m, m/f, or f/f. The Ruling Two don’t even have to be the married couple, so long as one of the Ruling Two are genetic heirs to the throne.
Take, for instance, Serena’s grandparents, her father’s parents. While they were husband and wife, they were not King and Queen. They were King and wife, with a secondary member of his support staff serving as the balance to his Ruling Two, (King and Dame, or King and Sir, in this instance, King and Sir). The Ruling Two do not have to be romantically linked.
Therefore, if Serena and Layla were to be wed, Serena would have the option to either nominate Layla as her Queen and the second half of her Ruling Two, or Layla (who honestly has no interest in learning how to rule a nation), could politely demur, allowing Serena to choose a more suitable candidate to rule beside her.
It is not the fact that they are a lesbian couple that prevents Serena and Layla from being together with ease, but that Layla is not only a commoner, she’s an Unlearned commoner (which is different from an uneducated commoner). Royalty don’t often marry for love, but for practicality, and Layla’s skills are impractical to assisting and supporting Serena in her future rule.