"Not that which goeth into the mouth defileth a man; but that which cometh out of the mouth, this defileth a man."
Fellatio is ok, but it is wrong to spit out afterwards.
I found out about this curious passage in the early 80s (horny teenage years), while reading a Penthouse, IIRC . It was in the Dear Penthouse letters section and went something like this:
Dear Penthouse, my wife is a devout catholic but refuses to give me oral sex. Do you know of a biblical justification that would reassure her? Mat 15:11
was the answer.