Your relationship with them is secret, they often exclude you from sharing affection, want you to have his baby for him without letting the kid or anyone else know you're the mother, and possibly give it up if your relationship ends. You clean the house, take care of their kid, and come here to ask if you have any rights to ask for what you want.
And you say you're not being used????
What you are to this couple is a maid/nanny/baby-making machine. They clearly sound like users. Odd that you can't see it. Do you really think they will let the world know the kid is yours once you pop it out, that they're in love with and fucking you, and that everything will be hunky-dory with a happy three-parent household? Seriously?
I bet if you started sitting around and asking them to do housework and watch their own damn kid, you'd be out on your ass in a heartbeat and they'd be out prowling for their next victim.
Move out and see if they keep up being in a relationship with you.