I don't think it's a slap in the face at all.
If Sakura is raising Salada when she is not actually her daughter, it's likely that she is doing so voluntarily. A Sasuke/Karin child would not have a stable place to live; they are both technically exiled criminals. Sending her to live in Konoha is a kindness in that case.
Not a slap in the face in terms of Sakura's kindness or her volunteering to care for a potential rival's child with her (obviously, in this case) old crush. I think Sakura would certainly do that.
No, a slap in the face in the way that it would mean that Sakura is shown 1. not as a medic, 2. not as a kunoichi, 3. not with her own child. It would be taking everything away from Sakura that she'd worked for and instead making her a nanny.
If it were to be the case, then showing Sakura caring for someone else's child is really a low blow, saying she's not with Naruto and she's clearly not with Sasuke either. If it were Karin's love child with Sasuke, that Sakura cares for, then we would know absolutely zero about the course of her life. I think her character already got the shaft by Kishimoto, but that situation would be an even bigger one.
It's bad enough that the child doesn't even look like Sakura. At all. (It's a visual emphasis on how important Sasuke is to this story that Sakura's child looks like the reestablishment of the Uchiha clan, not cute little pink-haired kids who will never be quite like the crazy Uchihas of old.)
But it's neither here nor there: I think these are their kids, the way its presented (albeit clumsily) in the manga. It doesn't make sense to for it to happen any other way. And the only true confirmation we have of this is because of the Sasuke/Sakura moment in the chapter before. That leads me to believe that Sasuke is with Sakura, thus Naruto is with Hinata. And it sucks all over again.
edit: I would love to see a thread about Naruto and Sakura's children. There's an adorable pic on slextrem's tumblr of a blond haired, green-eyed boy that looks I think I'm going to adopt into my headcanon NS family!