Quoting @batmansxrobin in https://answers.ea.com/t5/Game-Feedback-Ideas/Making-quot-spouse-quot-and-quot-partner-quot-labels-toggleable/m-p/13893839#M4423:
"The UI dropdown element for custom pronouns already exists. Changes and adaptations to it can be made. It would have made more sense for "custom relationship titles" to be added below "custom pronouns", and for them to be filled in by default for each she/he/they pronoun template like they already do... that way *all* gendered (like mother, father, sister, brother, aunt, uncle, etc) or neutral terms could eventually be included, and easily customizable at any time for each sim. Which is also so useful for personalization outside of gender too. What if you want some sims to be referred to by different names for mother like "mum", "mamma", "mater", "mama llama", etc? And considering they had to go update all the references anyway to be "romantic partners" or "engaged", it wouldn't add extra development time for it be a referenced string variable rather than static text. Want your sim to be both a "boyfriend" and a "wife"? Done. Use any terms you want. I do like that Lovestruck offers an option for custom relationship labels, but it's behind a paywall... so maybe that's the reason why it wasn't added as a simple addition to the dropdown, because that would've been free for everyone..."
(Just thought it made sense to include this suggestion here as well, since it's a potential solution)
