It is NOT a SIN for a Sikh to marry a non-Sikh IF HE HIMSELF DOES NOT CONVERT TO THE OTHER RELIGION AND HIS OFFSPRINGS FOLLOW HIS RELIGION. OTHERWISE ITS A SIN.

But here are the reasons WHY ONE SHOULD NOT MARRY A NON Sikh OF IT HE DOES WHAT PRECAUTIONS SHOULD THE PERSON TAKE. :

1. The Sikh should continue to his religion in all circumstances.
2. He SHOULD NOT at any cost change his religion.
3. He should persuade his partner to convert to his religion, if possible. Because Mother is the teacher of childrens if she is of different religion the childrens are bound to follow the said religion.
4. At any cost his offspring should follow his religion.

Reason : Guru has respected all religions and has advised us to follow our respective religions.

Thus in the instant questions there are following options one should follow depending upon the rigidity of his partner:

OPTION ONE : The Sikh follows his religion and the partner follows her religion but the offsprings follow the Sikh religion.

OPTION TWO : The partner to convert to Sikh religion.

OPTION THREE : The offsprings evenly follow the religion of parents. i.e if there are two children one should follow Sikhism and the other the other one.

BOTTOM LINE OF THE QUESTION IS :
Sikhism IS ALREADY IN DANGER THE EXISTING SIKHS ARE NOT FOLLOWING THE 5 K's OF GURU GOBIND SINGH. PLEASE FOR GODS SAKE INCREASE YOUR COMMUNITY AND DONT LET IT GOES INSTINCT. Sikhism IS THE MOST RECENT AND PRACTICAL RELIGION AND PLEASE PLEASE PLEASE PLEASE PLEASE PLEASE PLEASE PLEASE PLEASE DONT LET IT DOWN AND GET EXTINCT.