Ill try to not include any religion perspective and I appologise if I offend anyone. But this is my views.
We humans are at higher status than other living forms on earth. We could think, unlike other organisms.
Say, if people say sex is a thing that we can do when we want to do without any proper way(marriege) arent we just as same as animals?
Well sex is a divine well not divine..what is the word...ive forgotten but something to remember forever. It's makes to relations stronger and it bonds humans. Instead of making it like a normal thing why not cherish it in a way which you value it like marriege.
And like somebody say before this, if you are commited to someone than you can secks her or him. If you ARE really commited with each other why not marry her or him?
Sex is nothing without true love. It's only a desire.