Especially since the verse itself is so vague, that makes the most sense to me if it's not explicitly referring to premarital sex as the "immorality". My wife gives me some amazing perspective on my attitudes and actions in every facet of my life- she's helped me get control of my drinking and drug use, and overall anger. Shes helped me learn how to care deeply for another person. Why couldn't Paul be talking about this aspect of marriage? We had sex before we got married, and I don't really think that was part of my "immorality" as a younger man. In my heart it feels like that was perfectly fine.