I am a firm believer that laughter and enjoyment should be welcome in the bedroom.
My favourite funny sex story is: I was with my partner at the time and he had a particularly good impression of Gladiator that he was showing off earlier on in the day. I found that pretty sexy, what with the low gravelly voice and all. ANYWAY this got extended into while we were having sex. We were finishing things up with a blow job and as he came, with his hands in my hair he yelled ARE YOU NOT ENTERTAINED.
Granted I nearly died from laughing/choking/sperm in the nose but it's still one of the funniest things that's ever happened to me in bed.
Any other corkers?