A "crap shoot" is slang for an indeterminable outcome. A gamble, because you could win or lose. It could be bad or good. I assume this man wants to guess the sex of the baby, but isn't sure what the gender could be. If he is right, it's good, and no harm will be done. If he is wrong, he might offend someone, and that is a bad outcome.

I agree with Wendy's answer. It comes from the dice game "craps". It literally means a roll of the dice.