Is this the point where I stop being understanding to them? I've tried being very patient and not trying to take any moral high grounds or play any victim cards, even though I'd probably be justified in doing so.
You are yourself, and you should accept that they may not accept you right away or at all. Religion has deep roots that don't always disappear. If you just be yourself, and I'm sure that they'll see that you're still the child they raised. It's not like you're a drug addict or anything, so I would really hope they can get over it.
When I admitted I was pansexual (I worded it as bi, but whatever) I didn't change my day to day life at all. I still made coffee in the morning for my dad and I, and I still talked with them about the usual things. My advice is to not change a thing.
One last thing, a gay relationship should be treated just like a hetero relationship. Take your time to ease the person you're going to be with into you and your family's daily life. A solid relationship is a good thing, especially if your parents are skeptical.