I think Mariah has been basically spoiled and thinks she knows better than everyone.  Her reaction last season to the little mohawk kid getting arrested for being drunk or whatever the practical joke was was really embarrassing and over the top how she just went to pieces and freaked out.  I get they have their whole religion thing and standards that are expected of them and that reality might rattle their world if it actually happened but she really acted like a lunatic about all of it with her high and mighty stuff.  

It's funny to imagine her with sister wives, I bet she'd be a nightmare.  

I just think she's a bit of a spoiled dramatic kid, a couple of the teenagers act that way.  It's really none of her business what her mother wants to do and she is too old to be acting that way over something that will impact her mother in ways Mariah won't understand until she has her own children.  I get that Meri doesn't just want to plunge into it, it's complicated.