He means that you're gonna live forever - LOL :)
The song is about regret... and people regretting the things they didn't do more than the things they did do.... In this case, the "Good" person is less likely to sleep with the story teller, and he is arguing that if she were to die, she would have died without experiencing the full range of human experiences.... and that anyone who dies without those "Dies Young"
Good people die young when they are still innocent, I don't believe that to be true, I've heard children and seen children do cruel, cruel things.
Not true in my experience. More self-restrictive women in some Protestant and Jewish orders, I think; also those who have enough smarts to see through the testosterone fumes and know that he won't be hanging around in nine months.
Joel's lyrics come across more as a rejection of the "just say no" mantra common in some cultures, than as an attack on the Roman Catholic Church in specific.
Of course, it's also a fun tale of a common problem with horny young males, and a great caricature. I mean, a smooth talker like this? Just say no, Virginia, and knee him where it hurts if he doesn't get the message. Get out of the car and call a taxi.