I think labial tears are often overlooked in birth books/literature. I'd like to write a paper about them someday. I hope I get a chance to actually do it.
I had labial tearing during both my births. Though I was very well-educated about birth, I had never even heard of the possibility before it happened the first time, so I was very taken off guard by the experience. I gave birth semi-sitting and was convinced that if I'd stayed on my hands and knees like my instinct said, I wouldn't have torn. However, the second time, I did give birth on my hands and knees, but still tore again. The second baby was a pound bigger than the first and had a 15 inch head and was born in 2 hours total--the active phase of labor was about 45 minutes and I only pushed for a few minutes of that. I hypothesize that could possibly be related to the tears. I actually had purple bruises on my labia (which I didn't even know was possible either!). My perinuem was intact both times. I have photographic evidence of its very stretchy nature and it felt completely "normal" following birth both times--no pain or weakness, all the pp pain was centered at the front of my anatomy, not in the back.
The tear during the second birth was only milimeters away from the scar line from the previous tear, so I think either my pelvis is structured in such a way that it angles my babies out towards the front and puts pressure on the labial structure in a specific way, or, tearing the first time weakened my tissue and contributed to tearing the second time. I also had a second labial tear during the second birth that was lower down--kind of an inner "split" that didn't bother me at all and I could barely even tell where it was after the first couple of days. The other tears were in the clitoral hood and in the first birth, the top of the labia minora (one side is permanently detached from the clitoral hood now, which is probably why I didn't tear in the exact same place the second time, because it was already detached!).
Strangely, it took tearing again with the second birth to help me stop feeling "genitally mutilated" (which is genuinely how I felt) and to accept my "new normal" as okay. I didn't have the tears repaired either time.