Listen, 2,600 years ago the the prophet of the mazda made up some stuff where you can only marry other zoroastrians. Look what it got them. A whole civilization and half of the arabian peninsula down to 150k people. You can interfaith marry in christianity, islam, and hindi, do you see how many members of them live? They grow.
I also think if you interfaith marry you might be bringing in someone much more interested in the faith (not neccesarily God) than you. The other thing is sikhi is bleeding at the neck because of females apostatizing in droves. The panth needs families to come from somewhere.