The most common special instance during which I would discourage this practice is when mother, father or baby have an active yeast infection. If anyone in the family is suffering from a yeast infection it should be treated and resolved before oral-breast contact occurs.
Yeast infections are very common and easily spread. They can cause oral thrush or severe diaper rash in the baby, breast and nipple pain and vaginal yeast infection in the mother, "jock itch" in the father and athletes feet in any family member. It can be spread by sexual contact, breastfeeding, oral contact and on your hands. (Riordan, & Auerbach, 1999) It also survives on surfaces, like the shower floor, breast pump equipment, bath towels and burp cloths. It can be very difficult to eradicate. Once the yeast infection is cleared oral-breast contact can be resumed.
Also babies don't eat sperm.