Why saying gay is offensive?

This is dependent upon who is listening takes it offensively or not. For those that take offense, it could be because of sexual insecurity. To call someone gay when they don't know if they are or not, but claim to not be, will take offense because of repeated interpretation. So far they had to sit there and tell themselves, despite how they truly feel, that "I am not gay!" over and over again. Lying about ones self is very difficult and when someone intervenes and say "you are gay," offense is then taken.