Is Sex Without Marriage a Sin?
Only when you think it is a sin.
There is no answer to the OP in scripture or law because it applies to your own personal views.
The concept of 'sin' is a personal guilt trip.
It is based on your actions related to your beliefs.
If your beliefs do not consider it a sin, it is not a sin and you will experience no guilt.
If I am told and taught over my lifetime that eating chocolate is a sin.
If I eat chocolate, I have sinned according to my beliefs.
I bear he guilt of the sin.
I need forgiveness to remove the guilt.
Its a burden I put on myself because I believe it.
If I don't consider it a sin, I hold no burden for that action.
It is a non-event because it has no significance.
Many believe in the idea that they should not worship any gods before God.
Yet, they don't consider their need for money or material possessions a sin so there is no guilt.
Yet they worship these things openly without remorse.
It's not a sin, because they don't think of it as a sin.
No burden, no need for forgiveness.