Giving birth.
Why are we even talking about this?
Hell, I was knackered just from watching. My fiancée managed it without any pain medication, and she doesn't seem to remember the pain almost at all, but she felt it at the time I can tell you that.
The best logic presented so far is "They don't offer an epidural before you get kicked in the balls". No, they don't, but if they did, I'd probably take it.