In my case, I don't mean parallel to the ground. (Actually I'm riding a MTB/hybrid with flat bars.) I mean not bending your spine where it connects to the pelvis. I think (maybe this is where I'm mistaken) this is the way to ride, and it seems like I need to pivot my pelvis forward to sit in this way which in turn puts more weight on my perineum. I'm new at this so I'm trying to figure out the best posture to let me ride 4~5 hours relatively comfortably.

So then I was looking at photos on the web of cyclists, and noticed the roadies in the drops. Seems like with the handles lower then the seat and all, they gotta be rotating their hips forward. Or else keeping the back straight isn't as important as I think it is.