Why does God disapprove?

For example, I understand that the bible condemns premarital , but why? I comprehend all of the modern, trivial reasons like unwanted pregnancies and such, yet, it seems that devout Christians (my nan) believe it is wrong in every case and to hell you go. Exclude promiscuity, and think of a stable relationship with intent of being together forever. Why does God see this as a sin? What does marriage change in God's eyes in this case? Additionally, with cohabitation. Why are these socially inoffensive things considered sins, according to God? Will failure to repent result in said 'Hell'?