Dad : in furture, who will u wan to be ur wife? Son : usually, grandma dotes on me, so i will wan grandma to be my wife. Dad : rubbish! hw can my mum be ur wife? Son : :S son : den, y my mum can be ur wife? dad : @.@
OMG its about the son wanna marry the granny cuz his dad married his mom same as granny is the dads mom!
I think that he is referencing that he cannot decipher the poor typing. The joke would have been much funnier if it could have been read quicker and easier. But it was something of a struggle to read and completely killed the joke.
This 'style' of writing is just a sign of laziness, lack professionalism, or stupidity. I really can't see why anyone would do business with people like this. The quality of their writing is so bad, it reflects poorly on anyone working with them as well. The OP was right, I am laughing, just not at the joke.