Is this still because of the married couple tweet joke?

Orientation != mature, since kids can have any combination of parents/guardians.

IMO the guideline should be cheesy pun that would make someone’s kid of any age groan and shake their head, whereas, like others said, “uncle jokes” take a second more thought and are usually based on sexual (bodily and/or “action based”) themes. Maturity level isn’t so much the issue as is the difference between obvious wordplay and double entendres. It doesn’t need to be a stone-set line, but the genres are definitely distinct.