There is no philosphical basis to oppose a marriage between a Sikh and a Muslim. I am no scholar, but if someone can show where such is written I would appreciate it.
The question I guess really comes to one of identity, its definition, its creation and history. That is why there are so many tangents on this thread.
The Islamic invaders of India had a reputation for loot, rape, pillage and carnage. Therefore, most people obviously did not want that to happen to their women. Having said that, it is interesting that in 1947-48 the Pakistani and Pashtun forces in Kashmir did not have problems raping kashmiri women. The simple fact is that many people of India were converted to Islam not through simple choice but through force, intimidation and threat of savagery and rape and murder (sure, some did convert on their own but why would vast Rajput tribes of the punjab convert otherwise --- they still marry into their own castes even today, they did it out of fear).
So, there is no philosophy, only history. Why do the Serbs not freely marry with the Kosovar muslims and bosnian muslims? Why is there a struggle between Christians and Muslims in Sudan? Look at the bloodshed over Jerusalem over the ages. On a personal level, between two people, all of this does not matter. There is and has been a clash of cultures and religion for a long, long time.
I do think however, that these kinds of debates belong among the Islamic community and not small, secular, sometimes struggling minorities. It asks much of those whose identity and way of life are constantly under threat of assimilation to always employ and act upon their duty to be non prejudicial and non biased in the face of bigotry, in the face of bias, out there in the wide world. We as Sikhs already have a hard time holding on to our language, our traditions, our appearance and our identity. Muslims have no such problems, precisely because their ways and identity are clear cut, as is their definition of self and their general lack of ability to debate anyone on the merits of their faith, its actions, its weaknesses or its strengths.
This is not to say you are not welcome, only that you should understand where people on this forum come from and why they may react the way they do. We have no extremist or fundamentalist wing anymore (all wiped out with 1980s Indian government harmi Kutha style action on our people) whom we can comfortably rely upon to carry our traditions forward as the Muslims can. Therefore, the insecurity and angst are sometimes just below the surface.