I have always read this to mean coitus interuptus like Bill said, and the sin was that of direct disobedience and the family thing like Ganon said.
re: Adultry - I read a study once, which indicated that in the time period, women were more or less considered property. The sin of adultry was commited when using the property of another man, but in this study did not apply to "un-owned" women.
Here's a link to the page.
http://www.libchrist.com/bible/premaritalsex.html
maybe this is another thread altogether.
PS - I was also interested in, and did some of my own studying, and was unable to convince myself that oral sex is a sin. Some folks believe it is, and I wonder why ?