It would be helpful if you were to play the actual game, rather than relying on the Keep to role-play. The Keep is a method for importing your choices that you made in the actual games, not a real substitute for playing.
There are several factors involved with the baby issue, and I am afraid I have to get into some SPOILERS for the explanation.
Some background...
In order to become a Grey Warden, a person takes the darkspawn taint into themselves. This gives them a connection to the darkspawn and also the Archdemon. The main, final boss of DAO is the Archdemon, which has the soul of an Old God. When an Archdemon "dies," its soul jumps into the nearest tainted creature and the soul is able to continue on living. The only way for the Archdemon to be truly defeated is to be killed by a Grey Warden, because they are tainted as part of being a Grey Warden. BUT, the game explains that a darkspawn is a "soulless vessel," so it is able to contain the Archdemon soul, but a Grey Warden is not soulless, so when the Archdemon's soul goes into the Warden, there is a clash and both souls die. Thus, in order to ultimately kill an Archdemon, a Grey Warden must sacrifice their life.
In the game, on the eve of the fight with the Archdemon, Morrigan approaches the Warden PC and explains to them that she has a ritual (the Dark Ritual) that will enable her to conceive a child. Since this child will be conceived with a Grey Warden father, it will also be tainted. When the killing blow is struck, the Archdemon's soul will "seek out the child like a beacon" and jump into it, rather than the Warden who struck the killing blow. Because this new life is so pure, it will NOT die as a result. So, everybody lives!
Morrigan has her own reasons for wanting this child, which I'm not going to get into, but she does specifically want it. Saving the Warden PC's life is merely a bonus, possibly increased if you are in a romance with her.
But there is a problem for female Warden's isn't there? Since they can't impregnate Morrigan. The fail safe for this is Alistair OR Loghain, whom you can convince to sleep with her instead, thus providing the player who can't or won't do it themselves.
You do NOT have to have this baby in order to save the world. Your Warden can die, or order either Alistair or Loghain to make the final killing blow, allowing one of them to die in your stead. But if you have selected the option for your Warden to survive, and also have that Alistair or Loghain survive, that means that the Dark Ritual was performed.
The problem you are running into is because your character is in a romance with her. The baby choice does carry over into Dragon Age: Inquisition. There are two different babies that Morrigan can have: a normal baby, and an "Old God Baby" (OGB). If your character is in a relationship with her, she will become pregnant with your child regardless. The ONLY time when that baby is also the OGB is if you then do the Dark Ritual.
Now, I don't know how it works out in the game for a Morrigan romance play because I've never done it. However, if you are in a romance with her, and have been having sex, it doesn't make much sense for you to say, "I don't want to have sex with you on this one occasion, I'll go convince Alistair to do it." You "like to watch," and that is fine for you, but there are only so many role-play variations the game can account for. The Keep only has the choices that are present in the game.