Family traditions can be strange, especially when you're dealing with people from different cultures and different countries. That being said, there's also common courtesy and a certain amount of tolerance you should have for people who don't understand your expectations.
OP didn't handle it the way they parents would have liked, and now her parents and boyfriend think she's the AH. Here's the story in her own words...
My (f21) boyfriend (m24) of almost a year, invited me to dinner with his family. (Mom, dad and his 16yr old brother.) Never met them prior to that. The only thing I knew about them is that they’re conservative and christians but lovely people.
And they were. I got along really well with them, before dinner. They were lovely and talkative. When it was time for dinner, my bfs dad wanted to pray. After praying he said, something alone the lines of “let us dig and let the food keep us quiet.”