The fabrics thing pertains to the Hebrews. It's essentially saying "don't be vain by wearing showy stuff" not "it is a sin to wear 50% polyester." Still, I've met many orthodox Jews who take it seriously. Of course, they also do not mix meat and dairy simply because their scripture used the boiling of a calf in its mother's milk as a metaphors for sacrilege (wasn't literally saying they couldn't mix meat and dairy).
Either way, Christians aren't sacrificing doves and lambs and stuff while carrying around an Arc of the Covenant, so they know that the instructions for Hebrews/Israelites (God's "chosen people") don't quite pertain to them. If anything, not mixing fabrics means no fancy jewelry or make-up... nothing about interracial marriages. Are they going to refuse a wedding because the bride wore make-up and the groom had fancy gold cuff links?
It's just absolute BS. The Christian Bible and the Jewish Torah clearly support interracial marriage and no one deliberately misinterpreting those passages would be unaware of the parts that prove their interpretation wrong... which turns their false interpretation into an out-right lie...and that sin was so important it got it's own spot in the Ten Commandments.