Damn, that's a toughy. She seems like a really good girl & good catch so you wouldn't want to let her go, specially not with all the trash that's out there & specially not with the time you 2 have had together. It's not a simple break up & it's water under the bridge deal.
I would honestly just sit down with her & try explaining to her that it's not about the physical aspect of sex, but more the expression of love & care you guys have for each other. In other words, this isn't a "fcuk" & you are not interested in the physical (this is what makes pre-marital sex wrong). Make it clear to her that you do love her & you do care about her, more than anything else in the world, & that you guys aren't just having sex, but making love. Explain to her that you are so overwhelmed with feelings for her that this is your way of expressing your love for her to her, much like a kiss, simply more affectionate. Sex is the ultimate act of affection & there is nothing more. Make sure throughout all this to simply say, "If waiting is what you truly want, then I respect your decision & I will support you."
Some priests, at least younger ones that I know, support this train of thought. Maybe take her to see a priest with you to counter what she is being told by the extremist prayer group. NEVER say anything about her mom, or how her mom is doing this because she doesn't like you, etc.
Maybe, if you see this happening, tell her you do see yourself marrying her in the future if things continue the way they are. That being said, it isn't wrong or immoral for you guys to have sex. The point of not having pre-marital sex is to wait for that one person that you really love & care about. Truth is, you will run into more than one of those throughout your lifetime, therefore, pre-marital sex is outdated.
You could always use her extreme religious views against her. The church is also against contraceptives & other things of that nature, not just sexual. Find something she loves to indulge in, find how the Bible/church don't approve of it, & bring it up to her. Fight fire with fire :twisted:. Oh yea, make sure what you choose to pick on her for, you don't do

.