This one is too easy.. Morals (primarily in religion now) say that you shouldn't be having sex until you're married.
First of all, there is no REAL (as in well thought out philosophical) argument behind the idea that having sex before marriage is immoral. It may be against the so-called moral code
of one or more religions but that is irrelevant, because in a pluralistic society (not a theocracy) religion has no overarching mandate to assign moral or immoral value to actions.
In our democratic non-theocratic, pluralist societies, we generally only assign moral or immoral value to things which hurt people or restrict their right to express themselves:
-Killing someone because they stepped on your toe = immoral
-sex between adults with consent = moral or at least neutral
-sex between an adult and a child = immoral
Honestly, plenty of what went on in the bible would be considered immoral by our state and society today:
aka throwing your concubine out of your door to be gang raped and then cutting her into pieces:
Judges chapter 19 wrote:
25 But the men would not listen to him. So the man took his concubine and sent her outside to them, and they raped her and abused her throughout the night, and at dawn they let her go. 26 At daybreak the woman went back to the house where her master was staying, fell down at the door and lay there until daylight.
27 When her master got up in the morning and opened the door of the house and stepped out to continue on his way, there lay his concubine, fallen in the doorway of the house, with her hands on the threshold. 28 He said to her, “Get up; let’s go.” But there was no answer. Then the man put her on his donkey and set out for home.
29 When he reached home, he took a knife and cut up his concubine, limb by limb, into twelve parts and sent them into all the areas of Israel.
But I guess we should take all our morals from a 2000+ year old book of hideous fairy tales, amirite?