That night, she goes off on him for masturbating in bed with her, horrified and smiling in horror, and for the first time you can see that he's sort of delighted by the transgressive ickiness of what they did too. Which, it's very interesting, because breastfeeding is the most normal thing in the world, and the way guys are about breasts is about the least natural thing about breasts, and there's this very clear line between sexual use and everyday use, like with lots of your parts, that we don't like crossing, but out of all those, breastfeeding is probably the number one thing. The top of the hell no pyramid, as it were, and I mean, for about thirty obvious reasons. But it's also one of those taboos where nothing bad actually happens or could happen, so you get a lot of playing around with it, especially in stories like this. Poison River was basically 500 pages of this, and that's one of the best novels ever written. So I don't know.
So Nancy tells Andy to take a fucking Ambien, and he tries to explain how, for men, that's just one of the many great things about masturbation, and she's like, "I AM RIGHT HERE." Awesomely, Andy doesn't skip a beat: "Well? I'm engorged. It hurts. I need you to suck it out. Be the baby..." She grins to herself, but turns on the light and sits looking away from him while he lists all the weird shit they've gotten up to -- the bathtub scene, recently watching him fuck her sister, the suck & spit tonight -- and asks how, then, his masturbation in her vicinity is really all that weird.
Um, it's still weird. Don't jack off around other people. There is no amount of weirdness that can accumulate where that's normal. I don't care if it's wartime, or if your leg is chained to Cary Elwes in a dirty underground bathroom. I don't care if they started it.* Don't jack off around other people. It is a private activity.
(*Obviously, in that case, go for it. I am exaggerating to prove a point.) Nancy takes a less commonsensical approach, living as she does in the complete absence of an objective morality, and says the main reason they shouldn't go jacking off in front of each other is because they need to try and stay as normal as possible. That's what she says, but obviously what she means is that babies don't masturbate, and certainly not in bed with her. Andy's like, awesome, so we're just your normal brother and sister (in law) who share a house and a bed and a Mexican (-American) baby. That kind of normal.