Normal
What spiritual reasons might there be for a Muslim to marry a Sikh or a Sikh to marry a Muslim? Its just simply a non-starter. Some marriages are successful, others are not. Who is to say that a Muslim marrying another Muslim or a Sikh marrying another Sikh is guaranteed to be successful? All depends on the intention. If the intention of a Muslim is to marry a Sikh woman and does not forbid her from benefitting from the wisdom and grace of Babaji Guru Nanak, who is to say that such a marriage is wrong? It may be, in certain cases, that one cannot find the ideal husband or wife within your own community - and, in fact, marrying outside can shower more blessings and happiness than can be possibly imagined if one had married "within". So, it all depends on the individuals concerned. Baba Nanak's wisdom transcends such narrow, confessional, blind-spots and we should make double sure that what we are uttering is not simply the worst kind of ignorance imaginable.