I have a guilty pleasure: sexual tension and ill-fated romance in horror and apocalypse stories. So deep does my secret affinity run that it manifests as a major line in my own book (Savages). I simply cannot help myself.
Do not misread me; I am not looking for classic romance. I do not want a happy ending; I do not want courting or dating or any of that drama. Even if part of me is rooting for ultimate consummation or for the characters to end up together, I am always secretly satisfied when it goes so terribly awry. I think it is less about the actual romantic element and more about the juxtaposition of it within a terrifying or catastrophic scenario. It is normalcy in the traumatically abnormal.
Sexuality is also very primal, very base, which runs completely in line with survival, be it surviving a killer, the apocalypse, zombies, whatever. It seems appropriate to acknowledge and include that instinct while exploiting the others. It makes the scenario and the characters seem all the more real to us.
Humans are hooking up in every scenario; you cannot stop us. And when in real life does it ever play out like a romantic comedy? It is all the more accurate to be messy, ill-fated, or unrequited.
When I wrote my own book, the sexual tension between the characters is where the story began to blossom in my head. In a post-apocalyptic world slim on survivors, with all the normal world and distractions stripped away, I was able to concentrate on two elements: survival and her attraction to him. For me, the survival was the setting, and the attraction was the story.
And that is because of this guilty little pleasure I have. Clearly, however, I am not the only one, as this element does appear in horror and apocalypse stories.
For horror, Hannibal Lecter and Clarice Starling immediately come to mind, whether in the novels or the movies. In Silence of the Lambs, Hannibal is clearly intrigued by Clarice and her intelligence, in a similar way he was by Will Graham in Red Dragon. In both instances, he wants to toy with the other while also teaching them, minimally helpful manipulation.

However, his interactions with Clarice take on that additional level of sexual tension. Hannibal is aroused by her vulnerability, hungry for her specific psychological damage and idiosyncrasies, a level he never achieved with Will. I think this sexual tension and Hannibal’s attraction to Clarice is what makes their dynamic so interesting and convincing.
By Hannibal, Hannibal’s romantic attraction is fully realized and no longer relegated to simple sexual tension in their interactions. In the movie, he sacrifices his hand to spare Clarice hers; in the book, he drugs her and spirits her away to live with him in the jungle. By this point, it got a little too romantically centered for me but was still enjoyable.

The sexual tension in Silence of the Lambs was much more engaging and entertaining that the outright romantic pursuits of Hannibal, but throughout the franchise, that sexuality is a strong element between the characters and in the plot. Hannibal being my favorite fictional serial killer does not hurt either.
As far as apocalypse, (let’s go mainstream, why not?) The Walking Dead fully exploits the soap opera of human sexuality in a apocalypse survival scenario. The Walking Dead being such a sensation and its success bringing it so mainstream does always lead to more interpersonal drama, a tactic to entice outside the initial target audience. Yes, I have the zombie lovers, but if I have a little romance, let me hook those on the fence too.
The first instance of sexuality and romantic drama in The Walking Dead was the love triangle between Rick, Lori, and Shane. Rick wakes up to the zombie apocalypse and hunts down his family, only to find his wife entangled with (and impregnated by) his partner, Shane. Definitely soap opera worthy but enticing all the same. The scenario is also pretty realistic. If you thought your husband/partner was dead, would you not consider finding comfort with one another?

The relationship between Glenn and Maggie is probably the longest and most explored in the series. They start as convenient fuck buddies on the farm (sex always happens during lulls in combat, right?) then develop into a full romantic relationship. They get separated and reunited; they make horrible and dangerous decisions based on their love for each other. Again, this crosses my unrequited, inappropriate romance line (for my own personal affinity); however, it is still very effective. It gets the audience invested in them, rooting for them (and hence hooked on the show).

I, personally, think sexual tension and romance has a place in horror and apocalypse. It attracts my interest and also makes the scenarios and character seem more authentic to me. Even facing the end of the world, given a moment to breathe, I believe humans will continue to be sexually driven. Oh, it seems the zombies are gone for the moment; how about a roll around in an empty pharmacy?
However, I think the inclusion of this element must be applied properly. Too much or too idealistic and it violates the genre; too little and it is lost and its purpose is unrealized. It needs to augment the plot and play off of the survival scenario; if it takes over as the story, it becomes too much.
Hopefully I succeeded in doing just that in my own work.