In a futuristic Utopian city, built up in the sky, no form of institution like religion, nationality, marriage exists. It is strictly illegal for any couple to stay together for more than 3 months. A young couple almost end of their 3 months tenure found it impossible to part away and decided to flee down back to the old earth, a place only existed in their history books and which nobody has traveled from the city for last two centuries.

Your logline is really long. Loglines?usually have a 25 to 30 word limit. Yours is 80 but most of the elements are there. You have a futuristic/alternate universe situation which you establish clearly. The main characters are the couple. A goal - to escape.

I had a stab at it using the standard logline formula.
When something happens, the flawed character/s must do the action/achieve the goal or else something bad will happen.

In a future where?relationships lasting more than 3 months are illegal, a young couple unwilling to part ways?must escape their totalitarian society before they are imprisoned.

I think your story concept of lovers running away?isn't original, however the world of the story would be (I'm sure you have a million interesting details in your head). It is the kind of movie that I would pay to see because I love unique worlds portrayed on screen.

It's execution dependant so your script execution will need to be top notch.?