Maybe she does identify with their religion? I know people who call themselves Christians, but just simply never go to church, pray aloud, or read their Bibles. Some are more practicing, but are simply shy and very rarely pray in front of people. So depending on context (ie: if she told them she was religious) then it might make sense. Also, if they live in an extremely religious place where atheists are rare, they might have just assumed (not saying that's a good thing, but just explaining how it could happen). Who knows...

I grew up in a home where my dad would always pray before a family dinner. He never asked a guest to do it, and we had them often, so it seems a little strange to me too. Kind of putting them on the spot which could make them uncomfortable, which seems impolite for a host. I have faith myself, but I still don't feel very comfortable praying aloud in front of people. It's not so much a praying issue as it is a public speaking issue, and I'm sure many people who pray routinely feel the same way. I have a friend who's an atheist who works as a waitress in a very religious area, and people will sometimes ask her to pray with them before their meal, which seems super bizarre and awkward to me, but I guess it's very casual for some people. I don't think the boyfriend's family meant to make OP feel uncomfortable, but they shouldn't assume everyone is as comfortable with public prayer as they are.

This. They think it's polite like "Oh look see we think of you as a member of the family! Aren't you so happy we've included you in our ritual?" Thanks but no thanks. As the guest, you feel pressured to accept, and if you're especially not religious, it can just get awkward. They think they're being polite, but honestly it's kind of rude. Something that should only be done with guests you know are into that sort of thing.

"Thank you Jesus for this food. And Paco. And Pablo. And Julio. For working at dangerous less-than-minimum-wage jobs so that we can get this food at a reasonable price. And Steve, the stereotypical methhead trucker for abandoning his family for weeks at a time so that my lettuce is less than a week old when I get it from the Costco. Uhm. Bye." That's my prayer.

do you mean courteous?

I was thinking corduroys for some reason lol.

... and I was thinking something to do with curds, as in cheese curds. Ha-ha

You are trying to be the clever, well-educated atheist like Bill Maher. You're forgetting the clever and well-educated part.

Oh please, dear. I've never pretended to be educated. I don't even know who Bill Maher is, he's not important enough for me to care. And we both know I'm clever, your cheeky little comment isn't doing much to dent my self-confidence.

I'm sure OP would have mentioned if she deeply offended them or if they rejected her. I'm guessing she was just embarrassed and felt stupid for screwing up something so simple.