My husband is agnostic, I am atheist and we do not say grace. My father-in-law always says grace and insists everyone bow their head and participate. I have always respected this in his home, but I do not want this in my own home.
If you're religious and it's you household tradition for people to grasp hands and bow their heads while someone says grace, fine. But I've had people do it to me in restaurants and in my own house. No. Bow your head and pray quietly if you want. Don't try to force me to pretend to share your religion or my shrine to satan will start being the table's centerpiece. Being religious isn't an excuse to be an asshole.