Oh and the reason it's entitled 'mixed race' as I'm not sure if the issues are in fact cultural rather than religious iyswim as google doesn't show up that there anything in Sikh religion which forbids marriage outside the faith.
I think it is more cultural and also to do with individual families. Interestingly there was a discussion about this at work the other day, about half the department are Sikh and we were discussing whether it was racist to want your child to marry 'their own'.
Anyway it split between people wanting their children to marry within the community, for one person even to the village in India great grand parents came from, to the member of staff who has a non Sikh white partner and a child who is completely accepted by her family. We were also discussing white staff wanting their children to have a white partner.
Religious wise it shouldn't be a big deal, Sikhs see all monotheistic religions as different paths to the same end/purpose.
So I think the answer is it is more to do with family than culture, a bit like travelers marrying travelers and a white RC non traveler would be accepted by some, but not all traveler families.