Here is an interesting state of affairs.

Check on this:

http://www.aljazeera.com/news/2016/12/interfaith-weddings-uk-sikh-temples-161228112158801.html

One solution to the problem is that both be of the Sikh faith. 

However if the non-Sikh will not convert, then the temple can create a side temple as Christian Churches have a side chapel. Then the Sikh priest can solemnize the marriage for the Sikh brethren and his chosen un-Sikh bride at the side temple.

However, in spite of this, I am sure the purists of the faith will continue to protest.

THE PURE and THE RIGHT are famed 

for their bigoted rigidity and inflexibility.