I think part of the issue with these terms losing meaning is because we stopped using them as intended. Straight Gay and Bi were just the entry points in delivering vital info about what sexual organs you had and which ones you were engaging in sex acts with. Under old school definitions that would list me as Straight because I have a penis, and am only interested in engaging in sex with a vagina. Now, because it does not matter to me whether the person who has a vag is a man or woman, under the newer definitions I am bi. So the system made sense for the original system we set it up in, but its breaking down in our more modern way of thinking
edit: Side note, I especially like how this effectively makes me Shroedingers Gay, as I am either Straight or Bi depending on whos looking