Now if violence happens with no reaction then sex/relationships have the opposite problem. Many authors take an approach that the best way to stir things up is to screw with a romantic subplot. That is a good method, but you have to work towards it instead of hurling it off a cliff without warning. Part of this might be that you get an easy reaction with storylines that incorporate adultery, love, and romance of any nature.
A difficulty appears when one wants to shake up a romantic subplot on a whim. You need to plan it out or it will come off as shocking. Yes, you will get readers who still don’t see it coming because they are emotionally invested in the characters. So don’t expect to impress everyone. Still, set a few things up beforehand to make sure there are clues that it isn’t a rock solid relationship. For example, the characters can fight from time to time and each debate escalates in anger. If they break up or one cheats then it won’t come off as a last minute decisions. The same thing goes for when two characters marry. If they come off as hating each other then it won’t make much sense for them to fall into perfect marital bliss.
More importantly, you need to have the proper reactions in romantic subplots. If the characters are in an established open relationship then the reaction to ‘cheating’ shouldn’t be the same as if they were monogamous. In the same vein, a character who is cheated on shouldn’t simply walk away without a care and shrug it off unless they’ve established such a reaction to betrayal. Again, it’s very important to have your characters remain true to themselves.
There is another important aspect to the romance, which always comes up and tends to be mentioned more than another other part: SEX! This is a weird one because there is that joyous double standard that male characters can sleep around while female characters are frowned at if they do. Let’s not even go down this route. Putting gender aside, I think sex is different for every person, so each character should react in their own way. Some are anxious about it while others think it’s nothing more than a hobby. One part I always try to consider is if the character needs emotional connections for sex or they can settle for the physical. There are so many dynamics to this side of a relationship that I could keep going and still miss parts of it.
To sum up this rambling mess, authors should put some foreshadowing into a doomed relationship. This eases the impact and avoids making it look like a desperate attempt to shake the story up. Also, have the characters change from this event instead of remaining the same. Unless they were never in love or cared about each other in the first place.