I've taken enough coursework in mathematics to have double-majored in it (along with my physics degree) if I had wanted to, and at a tougher campus than you've ever step foot on.
If you think there was any mathematical misunderstanding in my post, then you didn't read it right.
If you only have one one-night stand in your life, then yes, you'll be unlucky to get a girl pregnant. (although your 1% number
looks too low: "The bottom line is that a single act of intercourse between a young couple has on average a one in 20 chance of pregnancy – this assumes the opportunity presented itself on a random day, as these things tend do when you are young."
However, if you have a large enough sample of one-night stands, then your overall chances of getting a girl pregnant in your lifetime are much higher. If they were 5% for a single one-night stand, then you have a 9.75% chance of having gotten at least one girl pregnant after two one-night stands, a 14.2625% chance of having gotten at least one girl pregnant after three one-night stands, and so on. Off the top of my head, the chances of getting at least one girl pregnant would break 50% after....29 sexual encounters. Of course, if it would be hard to call someone "unlucky" even if the chances were just 20% or 30%, because that's still a damn high risk for those consequences.
If you think I've made any mathematical misunderstanding there, then show it to your statistics professor and he'll correct it for you, because either you've made a false assumption or you're misreading my claims.