Thanks everyone for the helpful suggestions.
Rewriting my story as an original fic seems like a "when all else fails" kind of strategy, so I'll hold off on that one for the moment. Though I'm sure it would probably draw in more readers, as you suggest.
Well, as I said, the scene in the movie isn't so much about survivors hooking up as it is about 2 people who figure that they'll die soon anyway, so why not go out with a bang? Point of fact, the woman actually does get killed shortly afterward.
Of course, she survives in my fanfic. My fanfic initially extends the original scenario, so that the woman seduces the guy again before he can wander off on his own like he does in the movie. Then they get rescued.
Afterwards, they meet a month later at the funeral of one of their group who didn't make it and even though things are very awkward between them, they decide to hook up and spend the night together to comfort one another.
I wouldn't say it's romantic as there's a lot of guilt and grief between them and they don't really have much romantic chemistry, but plenty of sexual chemistry. She's basically just using the guy as a distraction and he's pretty much wrapped around her finger because she's so freaking hot. He's at war with himself a lot in that regard, but ultimately he winds up fucking her.
It gets complicated in other ways, too, because the dead friend was the girl that the guy was really in love with, so he feels guilty about having sex the night after her funeral, and unbeknownst to the man, the woman is pregnant from their earlier hook-up and she's debating with herself whether she should go through with it or get an abortion.