5 Know ye not, my son, that these things [fornication] are an abomination in the sight of the Lord; yea, most abominable above all sins save it be the shedding of innocent blood or denying the Holy Ghost? (Alma 39:5)
This quote from the Book of Mormon makes clear that fornication, adultery and sex sin in general are second only to murder in the eyes of God. I mention this here because I have heard active Latter-day Saints say that this is not so. Some have even ridiculed this teaching which has been reiterated by many presidents of the Church and members of the Twelve.
It is important that we as Latter-day Saints obtain our values from the gospel of Jesus Christ rather than from the secular world around us. Pop culture tells us a different story, but the teachings of the prophets are true and the mind of God. They are his mouthpiece or spokesmen here on earth. If we commit sex sin, we must repent and be forgiven, or we will be damned. Those who claim otherwise are simply wrong.