Whether or not someone is happy to give birth (which is not guaranteed) does not change that the physical process is very demanding in a way that can be harsh on our mental state.
Humans have evolved over millennia in ways that have ended up with one of the most invasive, body-intense pregnancies on Earth in part linked to the incredibly risky and intense labours and post-birth processes our bodies go through. It's well known for birth to sometimes leaving the mother with permanent injuries, disabilities, or dead, so it's no wonder those who are lucky enough to end up with far less are still often having an unpleasant and painful experience. We're sadly unlucky that we've evolved this way when many animals have it far easier at this in area, not so much in many others.
Some of the best things in life are risky, difficult, unpleasant to go through with the results being worth it -- a wanted birth is one of the most obvious examples of this and the idea that life includes suffering, even for what we want.
And, even having had 4 vaginal births with no more than slight labia tearing, I cannot say "beautiful" is a word I'd use. Have you seen a bed that drips or a floor with a puddle of bodily fluids? (I've given birth on beds, the floor, once on an upholstered chair). It's not a pretty sight, it's messy and unpleasant. Those things being worth it (particularly when you're not the one cleaning that mess 
) doesn't change that.