I believe that the death penalty is acceptable because there are cases where people commit these crimes and admit to these crimes and show no signs of remorse. They take innocent lives and sometimes even are serial killers. If they were able to, they would kill again with no problem!
Humans are hard wired to seek out mates in order to bear children and populate the earth. There is nothing immoral or wrong about premarital sex. Marriage is not an instinct, it is a man-made concept. Premarital sex is only immoral from society's point of view. It is completely normal from a natural point of view.
Premarital sex is immoral because the Bible tells us so. General society does not see it as immoral but, then again, general society doesn't tend to follow much of what the Bible tells us as truth. If one wants to be moral according to God's law they will not engage in premarital sex.