Some good responses here so far.  I love this topic, so I tend to write profusely about it. So please pardon the length.

This problem isn't confined to paleo.  Most anyone who changes their diet significantly from what their family eats goes through some variation of this experience.  Some families find it easier to tolerate or support these changes than others. 

Perhaps it will help to recognize how we establish our foodways beginning in the womb, and that they are not merely preferences, but also rituals, traditions, and entertainment. Foodways create a powerful link between our biological need for sustenance and our emotional need for connection, family and community.  Wherever people go, their foodways go with them. Food, like sex, is about as personal and intense as it can get.

A close friend in her third trimester talks frequently about the impact of her food choices now on how her future daughter experiences food.  She plans to take her daughter traveling, and she regularly chooses plenty of variety because she wants her daughter to enjoy foods anywhere they go.  She believes her prenatal choices (and later, while breastfeeding) can influence that.  She is reproducing her own cultural values before her daughter has even been born.  That's a lot of investment, over a long period of time.  Of course, not everyone has such adventures planned; still, one hopes most families take their children's food seriously.

Because foodways serve as a tool of cultural reproduction, any challenge to this can represent an attack (to use a melodramatic word) on one's culture.  I think many food interventions (e.g., attempts to address high diabetes rates in certain communities) have spotty success because they overlook the cultural importance of food and focus too much on medical issues and lists of forbidden things. 

What does all this jabber about culture and foodways have to do with your predicament?  I think we, as individuals,  and as individual families, often think of "ethnic" food traditions, and "culture," as something other people do.  What we do is just....what we do. It's easy to forget how important our own foodways are.  When we announce to our families that we no longer eat not just one food, but entire categories of food, without realizing it, we say "I am not like you anymore.  I am different.  I reject something that once bound us together."  We don't intend to say this, and I don't believe most people on the other side of the equation hear it in quite such stark terms.  But when mother can no longer provide baby with food, when mother's food is now poison, it's not hard to see how this gets under people's skin. 

Of course no one approach will work for everyone.  Some families might respond to the "trial balloon" approach, whereby one frames the change as a (perhaps temporary) experiment.  "I've been having some trouble, and I'm experimenting with avoiding certain foods to figure out what's causing it."  You've introduced the idea, connected it to your health (something most families will care about), and not framed it as a total, permanent rejection.  Over time, as the "experiment" shows results, the results will speak for themselves, and it becomes more difficult for them to object. Also they've had time to get used to the idea.

I also really like the suggestion another made here to give them one of the books on the subject.  This way they don't have to come to grips with your situation during a confrontation.  Another polite thing to do is introduce the subject in advance, and not at mealtime, when the stakes seem so much higher for everyone involved.

Another thing that might help is to do some role modeling: prepare a meal for your family that provides everything they could want while following your requirements. Don't just tell them, show them this is not an eating disorder, but a lively, healthy, completely nutritious, rich, satisfying (and not restrictive) diet.  Go ahead and make it one of those meals heavy on comfort foods that your family will recognize.  This isn't the time for livers and brains and shots of coconut oil (unless your family happens to love those). Don't forget a richly satisfying dessert--this isn't about calorie counting or austerity!

I would also avoid loaded language like "contaminated," and referring to the food others still eat as "crap."  Not that you would actually say that to them, but since you think of it that way, be careful not to inadvertently communicate it in other ways.  You wish they would appreciate your way of eating.   They need the same tolerance from you, perhaps more so, because they didn't choose this change.  I know we often get away with incredible (if unintentional) candor to the point of rudeness with our families, but when it comes to things like food, I think it can help to treat them as we would strangers or casual acquaintences: without too much judgment, and a willingness to overlook flaws in the spirit of getting along.  We wouldn't tell a stranger her religion was crap.  Don't be so sure that foodways don't carry similar weight and power to provoke strong responses. (After all, many food traditions are tied strongly to religious ones.)

Of course you may find that not all (perhaps not any) of these musings apply to you or your family.  No stranger on a website knows your family like you do, so any response will miss--or misinterpret--things.  But proceeding with caution and compassion seems like good advice for all of us.  Treat your family with love, and assume they are doing the same, even if you never agree on this matter.