I have been vegetarian my entire life – I was raised veggie by hippie parents and have literally never eaten meat—but I am definitely NOT offended when people eat meat in front of me. In general, I think it’s totally fine ettiquette to eat meat in front of vegetarians.
But, in this case, since you mention that they don’t eat meat for religious reasons, and since this is the first time you are meeting them, I“d recommend ordering a veggie entree. Think of it this way, if you were dating someone from an Orthodox jewish family, you would probably not want to order pork the first time you were meeting the parents. Or if the parents were classical musicians, you might not take them to a restaurant with loud rock music blasting ,even if that’s where you would normally eat. It’s just about being considerate and respectful of the tastes that your partner’s parents have.
It doesn’t mean you have to lie about who you are, but you don’t need to flaunt something on the first meeting that might offend them.
But if you were just going out to dinner with, say, some friends from college who are vegetarian? Order whatever you want and if they get all self righteous about it, find some other dining partners. And I“m saying that as a life-long veggie.