This is an interesting discussion.
I have a few questions about Catholicism, despite my Catholic upbringing...
irishpipes, you say "not attending Sunday Mass is a mortal sin". I assume that refers to holy days of obligation as well. What if you are too ill to attend or some other extenuating circumstance prevents your attendance? Is it still a mortal sin? I didn't think it was... (Although, with today's technological advancements, I'm hard pressed to think of an "acceptable" reason not to attend Mass. You can "attend" Mass remotely by watching a televised Mass or having your church call you and listening over the phone.)
It's my understanding that if you're conscious of a mortal sin, you must confess it AND do your penance before you can receive communion again - is that correct?
Anyway, to pull this thread slightly more on-topic, I took a look through the website Dee posted earlier. It seems that premarital sex is indeed considered a mortal sin because it falls under lust, one of the seven deadly sins. Also in that category are homosexual sex, masturbation, oral sex, sex where birth control is used... basically anything other than sex between a man and a woman who are married to each other, where the goal is to produce a baby. Whoof... did I mention I'm no longer Catholic?
(ETA: I'm not bashing Catholicism by any means, just indicating one of the many reasons why I did not feel Catholicism was "right" for me.)