I don't really care about Morrigan being super-irresponsible about secrets (who in Origins wasn't), but:
For the simple reason that she, Morrigan, is not a very good mother material which means the child is going to need his dad.
This is precisely why my Wardens (who never romanced her) would never do the Dark Ritual. Among many, many other terrible things, this woman thinks it's a fine idea to sacrifice a few dozen random slaves in a blood magic ritual. Regardless of how much she's touched by your Warden doing favors for her, (very, I'm sure) I'm not helping her become a mother, especially of something potentially powerful of which she will be the sole caretaker.
I am a little disappointed that doing the Dark Ritual will be revealed to be anything other than a very bad idea, but not surprised. NBD, I guess. Even if my Morrigan in Inquisition hasn't been "softened" by motherhood, I hope she's spent a little time this past decade thinking about why my Warden rejected her offer.
(I really like Morrigan as a character! She is v. fun! And evil! I don't think I can be convinced that someone who is upset if you don't help a demon possess a child isn't evil, but you are welcome to try, I guess)
They've streamlined it by tying it to the Goldanna and Marjolaine choices. I know meeting Goldanna or killing Marjolaine didn't automatically result in steeling the characters, but that's how they chose to address it in the Keep.
I'd also be disappointed if this is the case. I did both of these things consistently and rarely hardened either character.