I'm confused. The image in the comic may be what she means- she doesn't like it when HE is naked and yet is fine with him seeing her naked. Otherwise, how can it both be not a problem and yet make her uncomfortable? I wouldn't say that sex makes a marriage, but without it you're little more than good friends.
Now that said, assuming that she doesn't want him seeing HER naked, it might be because she's feeling uncomfortable with her body. He mentions a kid- if she just gave birth, she might not feel comfortable by not being her pre-birth size. Or it might be a large variety of things. If she doesn't want to see him naked, maybe she isn't as attracted to him anymore.
Regardless, it sounds as though it has reached the point that at the very least the couple needs to have a serious and sober sit-down and talk about it. Because when you can't see each other naked, sleep in separate rooms, can't have sex- there's a problem. Maybe you need a therapist. Maybe you need to redefine the marriage. Maybe she wants an open marriage, who knows?
But to my mind, the very first step is to take the initiative and sit down at the kitchen table. Have the kid over with a friend or a parent, whatever the case you need privacy for just the two of you.