If you're going to start picking and choosing which scriptural passages to live your life by, you're going to open a major can of theological worms. Rather than looking for "rules" in the bible, how about taking one of the most basic tenets of all religions - do unto others as you would have them do unto you - and letting that be a guide to your actions?
Just remember that a lot of biblical proscriptions fall into one of the following categories:
1. They had really good reasons when the Bible was written, like dietary rules and such. They were there to protect the tribe from eating things that were bad for you.
2. Cultural superstitions that were particular to the societies in which the Bible is contextualized, like taboos around menstruating women.
3. Man-made rules that people in power wanted to codify beyond challenge so they said "it offends God."
The whole "sex before marriage" thing makes a lot of sense when you consider that this was a patriarchal society without access to DNA paternity testing, and that the survival of your tribe might depend on passing on your property (goats, cattle, tents, water jugs) to your sons. It also makes a lot of sense when you consider that disputes over women (essentially considered property at the time) could cause a hell of a lot of strife that could put the welfare of the tribe/clan at risk. Hence the emphasis on adultery, inheritance, sexual behavior, etc.
Does your girlfriend want you to start a retirement account? Ask her to explain the difference between gambling online or in Vegas and putting your money into the stock market.
I would recommend an interesting book called "The Bible Tells Me So" that points out a lot of the contradictions in Biblical proscriptions of "hot topic" issues and explains a lot of the reasons why debating factions have chosen this or that scripture to support their point of view.
There are a lot of good, practical reasons to avoid sex before marriage and gambling, but "the Bible says so" is not really a logically sound one. (It also says to stone disobedient children and adulterous wives. And does she plan on keeping Kosher?) Definitely give some serious thought to your actions and their likely moral, spiritual and practical consequences, but remember that thinking for yourself is not a sin.