Matthew 15:11

"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.