My girlfriend feels dirty about the we have because of her religion.?

Me and my girlfriend of 5 months have pretty regularly now, and it hasn't been a big issue getting to this point. Pretty smooth sailing, neither of us pressuring the other. She expressed her desire to have before I did actually. She's a Christian, and I'm an atheist, I've been able to forget about it and have a happy relationship with her but tonight we had a long religious debate, the first we've ever had, and I've just found out that she believes herself to be a sinner, falling away from god, and that the we have is dirty and lustful. I love her so much, and I know she loves me, but shes suffering thinking that she's doing wrong, and I feel invalidated. What should I do?