As a moderator of parenting.se, I have to admit I was a little surprised to see this migrated, but I have to admit it makes sense.
While I have seen some specific communities encourage (limited) cross-posting of questions, the "official" SE stance is that questions have one, and only one, "right" community. If the question is ambiguous enough that it fits on multiple sites, it should be reworded to be more specific.
In all honesty, I think that this stance is a bit out of date. As more Area 51 proposals reach beta, there is more and more overlap between site. I've seen this issue come up between scifi.se and movies.se, writers.se and anime.se.
It also comes up between parenting.se and cogsci.se and even skeptics.se.
Here on pets, we probably have to be cognizant of any potential overlap with biology.se.
Until the pets proposal made it to beta, there was no overlap issue. As a result, parenting actually has a number of pet-related questions already, some of which would probably be more suited here than on parenting, in all honesty.
Ultimately, once we have our own moderators, we'll have more control over what does/does not get migrated (although the community team may override any decision our local moderators may make, if they feel it is necessary). However, I would let this stand as a reminder to us all that we should be careful to ensure that any question here be worded in such a way as to focus on the pet aspect of the issue.
In this specific example, yes, I think it could have been reworded to be more on topic here than on parenting. For example, something along the lines of "We're pregnant, and the doctor told us my wife needs to avoid litter. Our cats normally sleep in bed with us, and they're very affectionate. Should we keep them away from my wife? If so, how can we ease the transition for our cats?"