There are literally thousands of fetishes out there, for just about every conceivable thing. While this adult baby stuff is pretty alien to me, these people aren't hurting anyone - so why is it anyone's business? What people do to get their rocks off in the privacy of their bedroom is entirely their own prerogative, and I assure you there are fetishists who are far more deserving of scorn and shunning than a couple of adults who want to dress up in pampers and sleep in cribs.
Furthermore, I'm sure plenty of people here and elsewhere have fetishes that are just as "fucked-up" to other people as this adult baby thing is to you. It's an unhygienic and embarrassing fetish to have, sure, but like I said - it seems entirely harmless and I'm not seeing why it's particularly worthy of attention.