Tuppence is totally right about this one aspect of marraige: We do not just marry a wife, we marry into a family!!!
It's difficult enough when different races marry. It's even more difficult when those of different religious beliefs marry. The different families will pull at their respective children until one or the other surrenders. Then they are left with just the victorious family. Sometimes they isolate themselves from both families and try to make it on their own. Some families even kill their children when they marry outside their faith.
God's desire is for man and woman to be ONE in marriage and with HIM - spiritually, physically, emotionally, and intellectually. I do not speak for other faiths, but this is possible when Christians marry Christians.