Meeting my gf's parents for the first time?

My girlfriend and I have been together for 2 yrs, but I'm just now about to meet her parents since they live across the country. When they come over my girlfriend also wants to tell them that she is pregnant, the only problem is she says her parents are REALLY religious and don't even know she's not a virgin. On top of that I can be very outspoken and have a temper sometimes (which isn't a good combination), so if they say something crazy to me I will most likely tell them off which I would hate to have to do since I want to make a good first impression. I told my girlfriend how I feel about this, but she still wants to tell them everything now. Can anybody think of some possible ways I can try and hold my temper?