I have some... tough questions. My first novel (which I desperately want to be a series) wasn’t meant to have a romance element. There was going to be a main female character, two male supporting characters, and a female supporting character. I wrote draft one and decided to rewrite. In between rewrites, I had the idea to change the female side character. I daydream about this story extensively and had several ideas that I liked, one of them being the female, who I named Aster, being a different species meant to compliment that of my MC. The fact of the matter is, as I had tossed the idea around in my mind, Aster and MC became a couple. Their complimentary personalities and traits made it perfect and work wonderfully. I’m working on redoing my plot entirely. The two male characters don’t fit in as major characters anymore. Aster and MC’s relationship overshadowed the boys who, at that point, served little more than background comic relief that made them plain annoying. I don’t want it to be a “oh look how progressive I am for having a lgbtq couple in my story oooooo” Type of story. I truly believe I can get the best out of my plot with these two characters forming a relationship. Despite the relationship between these two characters being incredibly important to my main characters arc, I want the romance element to be in the background and incredibly slow burning. I want there to be chemistry, but I’d rather if this isn’t a one-book deal. Preferably the first book wouldn’t have the characters obviously forming a romantic relationship, get together in book 2, then continuing on through the rest of the series. I’m hoping for advice on that front (is it a bad idea, tips, how to avoid too much chemistry/romantic elements) Then there’s the lgbtq aspect. There are very little popular books with lgbtq couples. I looked at a list And didn’t recognize a single title, cover, etc. my concern is that this is an issue on the publisher side. Are they less likely to publish a series/novel if it has a same sex couple in it? And if they do, am I obligated to tell a publisher that I intend for the two main characters to fall in love if it isn’t obvious from the first book? Does this matter at all considering the romance isn’t a main aspect of the story? Sorry if this is the wrong subcategory, I was struggling to find one that worked better.
The question that writers have to ask themselves when trying to insert a romantic subplot is why? Why are you doing it? How does this subplot serve the larger story? Or is it just there because you like the idea and it does nothing for the overall narrative? Because far too many stories these days don't keep their eyes on the prize. They just throw a bunch of random stuff into the story that happen to appeal to the writer, but won't mean a thing to the reader. If the reader's not happy, ain't nobody happy. So make sure that spending time on this aspect of your story makes sense in the first place before you commit to it. I doubt any publisher, except the religious ones, care about the gender of your characters at all these days.
On the publishing side of things. I'd a reason those stories aren't popular is because they're about being LGBTQ+, like, it's a significant part of the story to show the characters struggle because they're gay or trans or whatever. This puts the story in a niche genre that only appeals to those invested in the LGBTQ+ community or those who can otherwise relate to the stories. However, I believe the Percy Jackson and the Roman off shoot stories by Rick Riordan have LGBTQ+ representation that doesn't become a major plot point (I haven't read the books in awhile but that's what I've heard about them recently). The reason those books were on a list of LGBTQ+ novels is because the LGBTQ+ element is a major element of the plot. If you want to have your MC get with another girl go for your life. You don't have to tell the publisher that there's LGBTQ+ in the novels. It's your novel, you can do what you want with it. Just ensure the new element fits in.
While I know very little on it as I have not explored it too much, Seems the way to press romance into the extremities, is to have them be apart with little time for intimacy in any form. While being in the military is easiest to keep the parties occupied on other things. Being in the medical field could make up the vast distance, with simply being unavailable being busy all the time. Granted a modern setting can only stretch the physical distance to being on opposite sides of the planet, which Sci-fi can fling lovers to opposite ends of the galaxy. So on the one hand you are 10,000+ miles and a plane ride away, the other 100,000 light years and anywhere from a few weeks, months, years, etc. depending on how fast they can travel weather they are FTL capable or not. Though I think for a slow burn is simply keeping the pair apart as much as possible, and/or preoccupied with other things, with little time for intimacy. The longer you keep them apart, the pay off shouldl be well worth the wait provided we like the characters enough to be pulling for them to actually find it more satisfying as a subplot element. As far as couplings in said relationship, I don't think it matters. (Unless one or more parties is a toaster, then it might raise a few eyebrows). Good luck, and have fun with your Novel series.