Why do people...

[Robo47]

say that they had sex with/raped someone else's moms?

[Mixfortune]

Mom jokes are... classic, but generally not well thought up. It's sort of the "quick get 'em an go" method of retorts, just a few rungs above calling people names based on sexual orientation but a tad below knock-knock jokes.

[Sord]

I usually just skip the dissing and kick the persons ass (almost got my own ass kicked out of school once http://www.pso-world.com/psoworld/images/phpbb/icons/smiles/anime2.gif )

[_Ted_]

On 2004-01-10 21:14, Sord wrote:
I usually just skip the dissing and kick the persons ass (almost got my own ass kicked out of school once http://www.pso-world.com/psoworld/images/phpbb/icons/smiles/anime2.gif )



Wow, thats something to be proud of.

As for your mom jokes, they're just a quick comeback. Its comeback used in an argument when one can't think of anything better to say. Its just below the very witty, "you are."
EX:
Person 1: Whoa, look the train.
Person 2: You're a train!

[-Luke-]

yer moms a train!

[Outrider]

Lindbergh's Baby's a train!

[geewj]

Err, take some time out your busy scheduel to read the rules and the stickys up top.