How do they know which guy bends over. Or conversely how do they know which woman is nicknamed Butch? Orwell was perhaps the most prescient man that ever lived.
They don't. It's really just a question of following the ad hoc usage of any given homo couple AP might be reporting on.
This is not a story about what homos do with each other.
It's a story about a news agency attempting psychological manipulation of the public.