If Mary had no labor pain, why does Revelation 12:2 describe her in pain?

You have caught something. This is why you cannot use Revelation 12 to prove that Mary had no pain in bearing Jesus. Revelation uses birth-pain imagery because that imagery runs all through the Old and New Testaments, sometimes literally and sometimes figuratively, as for the birth of Israel, and the symbolic book of the Apocalypse simply continues it. If you insist that woman refers to Mary, and you also hold that Mary had no pain, you have a problem, because there is no place in Scripture that says Mary did not have pain in childbirth. That is a Catholic traditional idea, but the church has given no dogmatic definition of it. Some Catholics even believe Jesus did not pass down the birth canal but simply popped out, and I would not hold to any of that unless the church herself backs it with a doctrinal statement, because we do not actually know whether it is true. Based on what Scripture says about childbirth and about Eve, having a child is not a painless experience; whatever natural mechanism moderated Eve's pain before the fall, it was increased afterward. So we had better be careful before declaring that Mary felt no pain.