I agree that LW1 is an unreliable narrator who wouldn't know a boundary if it bit her, and it appears to be biting her now. The child could have some health problem LW doesn't know about, or he could be in robust good health and his parents are cautious about contagious diseases because of the last 5 years.

There is a 3rd possibility. If either of the baby's parents has a history of sexual abuse, they might be very protective of their son, with hair-trigger reflexes around somebody saying they can't control how they express their affection, or "I love him so much, it just happened." I can see why the parents might want to make relatives demonstrate their ability to respect boundaries wrt the baby. And I can see why LW's son might not want to discuss his reasoning: the simplest being that if you have to defend a boundary, it isn't really being respected.