@Nermie01 Of the three strongest instincts the desire to procreate is actually the weakest, the other two easily override it, as it was already shown in an experiment. Fear and Hunger/thirst will always override Sexual impuses, always. In the middle of a war, while we do have cases of some perverts who rape people from the enemy country, we also have cases of people who instead of falling to these impulses, focus even more on just staying alive and do not engage in these activities or becomig violent while succumbing to anger. SO it's actually quite understandable that in his situation John might actually never get to feel any romantic feelings for anyone, thinking only on surviving the next day. It depends on the person.
But no, romance is not always good, and can in fact destroy a story, there are enough examples of that out there. One example I can easily give you is that Ange something anime, A Mecha anime about a princess that had a characteristic that is pretty much hated in her country and is sent to a prison only to become a mecha pilot. The story was originally serious and dark, and was completely annihilated the moment they added romance to it.