I'm with O my captain. This is not your problem--which is not to say it doesn't pain you, but that there's simply nothing you can do about it and you'll only feel lousy if you try and inevitably fail.
I will add a few things you can do if you want to make yourself feel less negative.
First, try to treat your parents with as much love and respect as you always have.
Second, never let them see the negatives of your relationship. Don't mention your minor fights, anything that irritates you about your boyfriend, etc, to avoid giving anyone ammo to fire at you.
Third, allow your parents to voice concerns you consider reasonable, e.g. if they see your boyfriend saying something particularly unkind to you. You don't have to discuss it in depth--say, if they ask about your sexual health--but it's understandable for them to worry that you're protecting yourself. You can respectfully listen, answer as you see fit, and then firmly shut down any line of inquiry if you so choose.
Fourth, if their concerns are unreasonable or voiced rudely, you can tell them you won't talk about the relationship at all. Then go back to #1, and have a nice conversation about something else.
If all that fails, then it sounds like family isn't as important to your parents as it is to you. Fundamentalists (of all stripes) aren't known for their flexibility, though I hope yours surprise you.