This is one of THE most common questions about unassisted birth. Every few months it seems someone starts a new thread about it.
Episiotomy causes tearing. Fingers inside the vagina and stretching the perineum causes tearing. Inhibition, by way of tenseness of the tissues and hindrance of the relase of tissue-softening hormones, causes tearing. Voluntary and directed pushing (because the pushing is happening before the tissues are ready for it) causes tearing.
Women rarely tear when they give birth instinctively and spontaneously and undistracted. I have never
heard of a deep tear (into muscle) in such a situation.
Small tears heal best on their own. Deeper tears that need stitches -- well, if it did happen I'd ask a midwife to repair it or go to the hospital. But the fear of having to do so certainly wouldn't keep me from UC, because there is virtually zero chance of it happening.