Nitpick nitpick: The post you were replying to said ‘future king or queen’- hence the ‘third in line’ comment is clearly referring to hypothetical future baby type people, not existing princess-by-marriage type people. Unless they were thinking Ms Cambridge is planning a sex reassignment, which would probably disqualify her from the throne as a wife of the monarch- though I’m not sure if there’s actually a specified law on it..
Even if that somehow went OK with the British public, Prince/Princess Catherine would then be second, and not third in line. 
What if the future king were to be found to enjoy putting on women’s clothing and hanging around in bars like a lumberjack?
well, if people would stop demanding every detail of every celebrity’s life, they would “leave those poor kids alone.”
Hey, I was just watching the news. I don’t buy any of those magazines or watch any of those shows that center around famous people.
It’s Pippa Middleton who’s pregnant and Harry’s the father.
Or is it Harry’s father who’s the father?
All of this is a royal pain in the arse.
Which is exactly what Pippa said before Harry got it right.
The rules haven’t been changed yet. Sixteen different countries all have to pass acts of Parliament, and in Canada it might take a constitutional amendment (& be approved by all 10 provinces). If she is pregnant with a girl she’d best not get pregnant again with a boy until all that’s sorted out.