Not normal == child abuse?
It's an unusual set up, but if the worse that happens is that they consider something normal that others consider weird, that's hardly a cause for alarm.
I'm with you on this one. Being raised by a single parent is becoming more and more accepted these days. I think being raised by 3 might actually be better than by 1. A single parent needs to do all the work, provide financially, cook, clean, take care of the kids (diapers, school, extra-curriculars). 2 parents is obviously ideal, but I'd imagine 3 is good too. 2 parents can work, the third can be more of a "maid" of the family, and kids get a ton of attention and care.
I don't see this "relationship" as being any more weird than hiring a full-time maid. Except in this case, its not only the husband who's fucking her, and the wife won't bitch when she finds out about it.