I honestly don't know how anyone who has read the Bible could argue against that interpretation. Although I don't think the Bible is inerrant, but if you're going to take it seriously at all...

Well, one could argue that these teachings are culturally dependent, and don't apply to the modern day, where everyone is expected to "Try before you buy" or else you'll have a heck of a time finding anyone to marry.

You could make an argument that finding a match is more important than maintaining purity in a culture that doesn't value it and where it is difficult or impossible to find.

One might even argue that sex where there is no possibility of pregnancy (IE using contraception) isn't actually sex. Is this what that other guy is trying to argue?

I don't know how anyone would argue that from a strict reading of the bible, sex doesn't imply marriage.

There is no concept of pre-marital sex in the bible. There is no such thing--you have sex, you're married now. The sin was never the sex, it's the abandonment afterwards.