Well, it could be that the descriptions or scenarios in the Naruto fanfiction are stimulating your imagination in a way that leads to those feelings. Or perhaps it's related to your own personal state of mind at the time of reading.
In a situation where a friend is stranded in a snow storm, it's a very serious matter. The cold can be deadly. Regarding the 'horny' part, it's not something to act on in such a dire situation. First, they need to make themselves visible to potential rescuers, for example, by using a bright cloth or reflective material. They should also try to keep their extremities warm. Survival is the key here, not any inappropriate behavior.
Horny fiction might be stories where the characters experience strong sexual desires or there are a lot of sexually charged situations. But this kind of fiction is not always mainstream and can be controversial. Some people might find it inappropriate, while others may see it as a form of exploring human sexuality in a fictional context.
It's a rather strange concept. In the context of the characters, it goes against the more typical relationships and characterizations we see in the original work. But in the world of fanfiction, people are free to explore all kinds of unusual ideas.