Nelson is bullying Bart by saying he's too scared to use a gun and has decided to also call him gay as an insult (by suggesting Bart wants to marry his best friend and by doing some flamboyant skipping.) He's expressing a pro-gun view while being homophobic.
Guns are part of the scene/storyline, and Nelson is pro-gun. Not all homophobes are pro-gun, so I specified that the one in this meme is because it's essential to what is happening.