It is my own thinking--
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/hard-talk/8550-how-many-sikhs-have-married-out.html
A SIKH shod not marry a Non SIKH girl, b'coz it will create lot of problem for the couple and as well as for the children. Couple will be of different cultural background so it will lead to difference in thoughts. I was in a same situation but in the end I quit my friendship. I supported my decision on these points- she dint know how to worship(PATH), rules, norms of SIKHISM. So how she will convey the message of our GURUS to our children. how they will come to know that we have such a rich culture.