"Know ye not, my son, that these things (sexual sins) 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) 
 
You are correct sexual sins such as fornication and adultery are very serious sins in the eyes of the Lord only greater sins are murder and denying the Holy Ghost (the unpardonable sin).  
 
The reasoning behind this is that God made man and women to be wed and to use the gift of sexual relations to bring children into this world so the spirit children of God can come to the Earth and progress and learn.  
 
Sex is a life bringing gift and when abused for pleasure it brings many pains and misery to all. 
 
Abstaining from any sexual contact until marriage and only having sexual relations with your spouse after marriage is the only righteous relations in the eyes of the lord.

Is the idea that murder is unforgiveable doctrine within the LDS Church? Or is that just a view of some members?