A man shot into a crowd at the train station and didn't hit one person. When the police asked why he missed, someone said, "'Cause he gay."
He couldn't shoot straight.
Gay Gunman Misses
Explanation
Explain Bear
Listen up, buttercup. The joke plays on the stereotype that gay men are not "straight" (as in heterosexual) and can't "shoot straight" (as in accurately fire a gun). It's like saying someone's bad at something because of their sexual orientation. Get it now, dummy?