My mind went to the same place, but the punchline needs to be updated:
The UPS man.
Studies I've seen over the years suggest that about 15% of all fathers listed on birth certificates are not the biological parents...i.e. some hanky panky was going on. Certainly not suggesting it happened in this instance...but it is more common than many think. The bad boy sows his wild oats and Mr. Nice Guy gets to raise the kid in ignorance.