Maybe the simple in-story answer is that she wants the child, and that she actually loves Rumple, no matter how misguided the sentiment might appear from the audience’s perspective. Additionally, if you recall the episode, she essentially tells Rumple to go and fix his mess with Hades — which suggests that, for all intents and purposes, Belle actually trusts Rumple to put up an effective resistance to whatever Hades’s plans for the baby might be. Since we’ve seen Rumple literally move heaven and earth for his offspring, I’d say that trust isn’t entirely misplaced.
I think your question conflates two things: Rumple’s viability as a partner/father, and the likelihood of Hades getting the baby. I don’t see the latter as a foregone conclusion.
You can have another child with a sane, non-socio-path who doesn’t fraternize and make deals with demonic forces whilst bringing havoc on innocents and constantly putting you and your family at risk while you claim to “love” him.
On OUAT, “bringing havoc on innocents and constantly putting you and your family at risk” seems to be something that every single main character has done at one point or another. This doesn’t seem to hurt their reproductive chances. In that logic, why didn’t Robin and/or Regina force Zelena to terminate the pregnancy? She’s certainly a danger to herself and others.