"I'm pretty sure my father would have wanted to know why you aren't sticking with your own kid, actually."
-Harry Potter (DH11)
Lupin, worried that his expected child with Tonks will be a werewolf and feeling guilty for starting a family despite his outcast status, comes to Number Twelve, Grimmauld Place and tries to convince Harry to let him join in on the quest. Harry gets angry at Lupin for abandoning his family to go on an adventure, and the two argue. (DH11)
Harry is understandably concerned with he hears someone entering the house – especially since a couple of Death Eaters had been hanging around Grimmauld Place all day. Their visitor turns out to be Lupin, who brings Butterbeer and a copy of the Daily Prophet. The first thing Lupin does (after proving that he is who he says he is to Harry and chiding Ron and Hermione for letting their guard down too soon) is catch Harry, Ron, and Hermione up on news from the outside world. He reports that Death Eaters and the ministry are breathing down the necks of everyone in the Order, but everyone is safe. He also explains that the Voldemort-controlled government has started rounding up Muggle-borns and has been intimating that Harry had a hand in Dumbledore’s death.
Then Lupin suggests that he join his former students on their quest, providing security during full moons. Harry doesn’t see how that could work, and tries to figure out why Lupin wants to join them. Lupin reveals that Tonks is pregnant and that he is terrified that the baby will be a werewolf and he feels guilty for starting a family despite his outcast status.
Lupin says that James would want him to join the quest, but Harry gets angry, calls Lupin a coward, and tells him that James would want him to stick with his wife and their child. Emotions rise until Lupin suddenly leaves. After Lupin is gone, Harry feels guilty for being so harsh, but reasons that Lupin was acting like a coward and the fight might have been worth it if it makes Lupin go back to Tonks. (DH11)
This happens earlier the same evening Kreacher returns with Mundungus.
Lupin's guilt and fear are even more heart-wrenching in light of the fact that Rowling intended Lupin's experience with lycanthropy as a metaphor for life with real diseases that carry stigma, specifically citing HIV/AIDS as an example (Pm). -BB
Rowling goes on to state that wolves are very concerned with their family groups, but that Lupin is (understandably) repelled by everything having to do with wolves (Pm). -BB
When Lupin tries to argue that James would have wanted him to join the quest, Harry firmly disagrees, believing James would want Lupin to remain loyal to his child... A little bit like Lily willingly sacrificing herself to save Harry or like Narcissa lying to Voldemort to protect Draco. -BB
The fact that Lupin thinks his child would be ashamed of him is a testament to the intensity of the external stigma and his own self-loathing, considering his support for those with Muggle parents. -BB