Romance and sex are perfectly normal parts of life. This means that it’s quite reasonable to have them in our fiction; of course, they don’t fit into every story, but this is for those into which they do.
It’s a good idea to have a personality profile at least loosely sketched out before you embark on this little journey. You’ll find that it makes things much easier. Why? You need to understand your characters to be able to write a convincing and enjoyable romantic sub-plot. I wouldn’t be a very good editor if I encouraged my readers and authors to write shallow, nonsensical, trope-filled romantic sub-plots, now would I?
With all that out of the way, we’ll look into romantic sub-plots. As the name suggests, a romantic sub-plot is a sub-plot, which is to say a story arc that occurs alongside and in the background of the main plot, that focuses on a relationship. Usually it’ll involve two (or more) people coming together, but it can, of course, follow the arc of a more established couple.
I should add at this point that this guide can be applied to romance novels, too. The thinking will be similar, only the romance itself will be the focus.
First, you need to know what exactly your characters look for in a potential partner. You’ll know this from your personality profile. This will mean that you’ll know what the spark is between the pairing. Does he fall into his deep blue eyes? Can she not resist the way she’s so selfless and helps the puppies? Whatever the spark and attraction is, that will give you an idea of their initial connection. For example, if they’re both space marines out on Zenari fighting the giant slugs, then she could grab his attention with her salt-shaker weapon adaptation. It was something she made in her down time, and he can’t get enough of hard-ass women who know how to use their hands and some tools. She loves a brawny man who just wants to save the world and appreciates her handiwork while having a caring side.
Once you have the spark and the meeting down, you can start exploring their actual compatibility. Are they both after the same thing? Could it be that one thoroughly enjoys having multiple partners, whereas the other wants long-term monogamy? That would lead to external and internal conflict. On the one hand, they’re wonderful and potentially worth being monogamous for, and they don’t want to hurt them, but… having fun and experimenting with new people is important to them. Maybe they’re just not ready to give that up. That would bring about the internal battle, debating if the monogamous partner is worth it, as well as the external conflict between the pair as the monogamous one tries to change the other.
They could even be wonderfully compatible and made for each other; it happens in real life.
Now, keep in mind that this romantic sub-plot should aid the main plot. It must add something, and help the main plot move forwards (unless it’s a romance, then said romantic plot is the main plot). If it isn’t integrated into the main plot and given a purpose, then it becomes nothing more than a distraction. For example, the woman could save the man from impending doom at the whim of the killer slugs with her salt-shaker adaptation; he’s so impressed that he shows his superior and the war is won! It doesn’t have to be quite as big and grand as that, but the point stands. The actions within the romantic sub-plot should have some purpose, some meaning, and some value.
Now that they’re together and they’re at least somewhat compatible, you should consider just how long they’ll last as a pair. Were they only ever destined to be a quick fling? Do both parties agree on that? If it’s one sided, then that adds more conflict and more potential plot-points. The upset party could go ‘bunny-boiler’, or become despondent and lose their job. If they do remain together, then how does that affect them as characters, and what impact does it have on the plot? How do secondary characters feel about and react to it? Could it be that character D had been hoping to marry character A, but character B stole them? How will character D react to that? What plot points come about thanks to that?
This is the really important bit – stay true to your characters, your world, and your plot. If you take a previously flighty, polyamorous party-boy and make him settle down with some meek boy who’s never touched a drink in his life, there’d better be an amazing reason for that. There had also better be a lot of conflict and character growth on all sides. It cannot happen overnight – it wouldn’t in the real world. If they’re in an incredibly stressful situation, and the world and/or plot puts them in a position where they’re not sure if they’ll live to see the end of the week, it could be a different thing. High-stress situations can bring about unlikely couples, purely because people are social animals and the comfort of a romantic and/or sexual partner is something people crave.
Your romantic subplot needs to make as much sense and work in the same way as your main plot. There needs to be a reason behind and purpose for each action and reaction. You can’t have something come out of the blue and shrug your shoulders with a ‘just because’. There has to be some basis somewhere. Ideally, it should also form a similar arc shape (a la Freytag’s Pyramid) to the main plot if possible. It is, after all, a subplot, so the ideas behind the main arc apply here too.
In summary, understand your characters’ personalities and stay true to them. Don’t make them do something completely out of character unless your world or plot gives you a good reason for it. Make the most of your sub-plot and the conflict it generates. It can be a fantastic tool.