Along with actively guarding one's mind and heart, a man must be actively seeking to become "the husband of one wife" (1 Timothy 3:2). Literally in the original Greek text, this is the idea of being "a one-woman man." I as an unmarried young man must realize that, more likely than not, I will one day be married, and I am to do my wife good and not evil all the days of my life. I must be seeking to guard my heart for her, and not piece-mealing it out to just any attractive young lady that walks by (Matthew 5:27-28). Even among godly young ladies, I must actively be seeking to treat them "as sisters, with all purity" (1 Timothy 5:2).
May God strengthen us all and deliver us out of the snares of the immoral woman.
[Recommended reading: J.C. Ryle, Thoughts for Young Men; available on Amazon.com, VisionForum.com, CalvaryPress.com, and other places.]