The wall isn't swollen, giving away that there is a human being inside it. An identifiable human being that we don't need to see as a finished article to identify, thanks to the internet informing us "so and so continued to film while pregnant with her first child."
That human being isn't identifiable without any external input, you admit as much yourself. That input isn't any more or less available with any other type of porn. So again, if the child is behind a wall of flesh, or a wall of concrete 50,000 miles away, or is born ten years after the scene is filmed you still have the potential for someone being taunted with the material who had no say in it's production.
That is contributing, by virtue of being inside their mother, to the creation of a "pregnant girl gets fucked" (for instance) scene. For guys that have that kink, to masturbate while watching. Think about that for a second.
They're not involved with the actual act. You know this, Stiffy. You wouldn't argue that a man having sex with his pregnant wife is committing incest, would you?
Taking that into account, while they may (in an unidentifiable and indirect way) be appearing on screen, they're not involved in the act, just in the production. Now, consider for a moment the film
Antichrist with William Dafoe and Charlotte Gainsbourg. There is a baby in that film. There is also penetration. If you can agree that a man having sex with his pregnant wife is not incest, and thus that the baby is not involved in the actual sex, the argument simply becomes an issue of the child being involved with the production itself. Thus, logically, you'd have to have a problem with
Antichrist since it features both a baby and penetration, but that wouldn't seem very logical (well, I'm sure there's plenty of valid issues to have with
Antichrist, but I don't think that's one of them).
Logically, I just don't see your argument. I can see your misgivings, and I share them (since it
is kind of squicky... and I'd file in under the same heading as porn that is or is meant to look painful, or porn involving people who look underage but aren't) but ultimately I don't think that's a valid reason to suggest it's something immoral, just controversial and... icky for a lot of people, myself included.