My buddy's fiancee wasn't allowed to play a Space Marine with the rest of us. Because she was a girl.
So while we all got to roll up the mightiest protectors of the Imperium, demigods of war, striding across the battlefield and bringing glory and honor to the Blood Angels, Ultramarines, and Space Wolves...she got sat down with whole different rulebooks than the rest of us, playing a compatible game from a different game line, and had to make a sexy, skinsuit-wearing, agent of the Occicio Assassinorum, instead. Had to play a whole different game than us, at a whole different power level. Missing special rules we had, missing out on her every attack tearing apart whole units of enemy bad guys, missing out on power armor, inhumanly high attributes, and being part of the team. All of it, she missed out.
That game didn't last very long. She didn't do very much in it. She never wargames with the rest of us, other times we hung out.
I think about that a lot.