During a recent email exchange, a friend and I discussed how much we enjoy reading about characters who are tightly focused on saving the day in (non-erotic) sci-fi romances with some kind of danger component or important mission. In other words, regardless of the characters' ages, the couple conveys an ability to delay sex-related gratification when danger looms.
For me, dangerous situations/missions are at distinct odds with feeling sexy and relaxed. Often time is of the essence, a villain is on the couple's tail, and/or the characters get seriously grimy. Such situations don't seem conducive to nurturing the physical aspect of one's romance, nor do I expect them to.
Many of these stories feature characters who have an important job to accomplish while enduring unpleasant physical situations (e.g., lots of dirt, heat, sweat, alien bugs, injuries, etc.). Yet sometimes one or both of the couple will engage in mental lusting or even sex at a time when I'm thinking, "Shouldn't they be more focused here?". When a couple's lives and/or the lives of other characters are at stake, I bring an expectation that sexual thrills will either already have taken place or be delayed until after the danger/mission has passed. If the couple doesn't seem to be worried about dying or preventing a catastrophe, becoming distracted by sex risks devaluing whatever it is they're trying to save.