Where did she use the word Sin?

No, but considering he'd now be buried in my back yard, I'm not the one he'd have to worry about forgiving him.

Being married is not a
guarantee that any of these
scenarios couldn't and wouldn't happen.

No but would help a lot with it all.

Disease - There would be no sexual disease if no one ever did any immoral sexual actions as in but not limited to, multiple sex partners, sex with animals, and much more. If those immoralities were not committed, STD's would not exist.

Low self worth - Neither partners would ever have low self worth, for their spouse would make them feel as though they were worth all the money in the world. Marriage is more then just a piece of paper or a concept. A marriage is actions as well. If two are married, they will act as if they're married. This includes but not limited to taking care of the other physically and emotionally ensuring their happiness.

Fatherless children - No, sex after marriage does not ensure this, for this can be caused by many different elements including but not limited to the dad passing away. But this is more prevalent in sexual relations outside of marriage, this occurs more often in that form of relation then between two married people.

Abortion - This to is more prevalent in sexual relations outside of marriage. For people become married to make a family, to make a family one would want to have kids. Thus no abortion would be taking place. This happens more in relationships where the two are not married. Unintentional pregnancies.

In the title of the thread:

is having sex with your girlfriend a sin