Why doesn't Belle just get an abortion?

Or she could work together with Rumple and the rest of the “heroes” to find away to make the contract invalid.

Okay, from a Once-verse perspective:

Emma, the DARK ONE, chose to accelerate a 2-month pregnancy to term to avoid harming the child of the woman she intended to murder. It’s clear that in the magical morality system of the show, whether or not it’s the same one you have, and we really REALLY shouldn’t get into that discussion (there is no happy ending in that debate), the baby is considered a person from the moment of conception. That’s why we can have prophecies, deals, visions about them etc.

As my sister said, the going method for getting rid of an unwanted child in this universe is to leave it by the side of the road (or let the Shadow take it/put it up for adoption/send it through a wardrobe…).

Belle is about the last person on the show who would even consider terminating a pregnancy. And LOOK at their faces when Rumple tells her. This is a child that is VERY wanted, by BOTH its parents. Belle isn’t even going to consider killing it, both because of the show’s morality structure and her own character.

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.