Hi Jem, also remember, that (for some reason) the Y chromosome is genetically weaker, so in some instances men will only have daughters as the XX is more robust than the XY and results in viable pregnancies. The sort of example I'm thinking of is Airforce men who work on the airfield or with electronic warfare things, the radar waves interfere with the Y chromosomes so they all end up with lots of daughters but no sons. I have never heard of a man only producing Y sperms as they are formed from his own genetic material which divides equally into X and Y, I would imagine that is biologically impossible to only have Y sperms.
:-)