1. A woman is grossly unattractive.
2. A man is so unattractive/ineffectual/impotent that he could in no way father a child claimed to be his.
3.A woman's promiscuity brings paternity into doubt.
2. Yeah right, it's his! Must have been a spoon job.
3. She'd have more chance of knowing who the father is if it were a spoon job.