[boring head on] I reckon it's more to do with protecting your own sexual health and thus reproductive potential. You're more likely to detect a minging minge with your taste buds. Giving a girly the big "O" could just make her more cock hungry, and not just for yours. I site the muzzie fundamentalist practice of female circumcision as supplementary evidence.
The second bit about sperm retention has some merit, but it relies on the hypothesis that you'd be willing to lick it until your tongue drops off after you've shot your load - not a particularly credible theory surely?
I did know a girly who died of cock sucking cancer, a tragic loss that still haunts me. You should advise your daughters to spit, not swallow.