I find that to be perfectly understandable. Nothing is spelled out precisely for us in this film. There's an ambiguity to the characters' relationships that could easily cause different people to form different opinions about what happened. It's purposeful, and an interesting concept.
Two old friends (or possibly former lovers) go on a camping trip and are having a rollicking good time. Then they pick up hitchhiker. You kind of wonder when the hitchhiker is going to turn out to be an ax murderer. Especially after the gay guy asks if his being gay is a problem for the hitchhiker.
This is a slender film that at first glance doesn't seem to add up to much. However, there is dynamic tension throughout the latter part of the film for reasons we're not quite sure we understand. The relationships are purposely somewhat vague and ill-defined, something I found to be an insightful take on modern relationships, especially gay relationships.
My initial reaction to the end of this film was WTF? On reflection, I get it. I liked this movie. It's deceptively simple-looking but there isn't anything simple about it. You know, like life.