Ending *SPOILERS*
It seems to me that many people when discussing the ending are forgetting about the huge role that the environment/scenery plays in the film. Franck is literally "lost" in the woods by himself at the end, because his desire and infatuation have led him down a dangerous path, and Michel, representing our destructive tendencies and temptations, is going to kill him. He calls out to Michel still because he cannot help himself; no matter how hard he tries he is still caught up in his temptations and infatuation. He uses the word "love" several times when talking about Michel, but since we know the film takes place in only ten days, what Franck thinks is love is actually more like being smitten or infatuated.
Henri's character represents the meaningful and deep, but nevertheless blandness of everyday life and safety/security. We as the viewer know Franck should be with Henri - it's the safer and more meaningful choice, since they connect on a deeper level that goes beyond just carnal pleasure. But Franck consistently leaves Henri by himself or breaks off plans with him to be with Michel, who is not good for him but provides him with thrills and excitement.
While the film is very sexually charged, I think people lose sight of the fact that sex here is really just used as a metaphor for all of life's temptations and human beings' self-destructive behaviour. You could easily take out sex and insert any number of thrilling but destructive behaviours here and the meaning would be similar. Whether it's skydiving, mountain climbing, boating, drinking, smoking, drugs, etc., humans constantly engage in behaviour that is destructive to themselves or others around them, and they do it merely for the thrill or physical pleasure it brings. This movie asks some very meaningful questions about why we do that, and why when we are crazily pursuing a particular goal or activity that we usually don't stop to think about the potential consequences. We are so often willing to roll the dice and risk it all for a temporary physical pleasure or thought. What makes the movie so great is that the director shows there are no simple answers. If we lived in the safe and secure way every day life might have no excitement, but if we live only through pleasure and adrenaline life can end very quickly or have no meaning, since any thrilling behaviour becomes commonplace if done enough.
The movie's setting in a "cruising spot" is a perfect one because it captures the dichotomy of men (or anyone else for that matter) metaphorically pursuing both pleasure and deeper connections, with the line between them often being blurred.
Don't spend too much time wondering about what happened to Franck in the end, because it's not about what "happened" to him physically, or whether Michel captures and kills him; it's more about the path Franck's choices have led him down, and most importantly, why his desires have overridden his rational thoughts and what that says about human nature.