Need opinions on an important issue with my fiance

There's a lot of players and unknowns in this, so let's get to it.

First, the parents. Religion aside, do they sincerely believe that not cohabiting is better for a successful marriage? Do they approve of you? In their minds, are they putting your fiancee (and you) first? Maybe they think, "Hey, we didn't live together beforehand and it worked out great for us." If any of these possibilities are true do you really want to say to your fiancee, "Hey, I know those people unconditionally love you and all, but I'm putting my foot down and telling you this is the 21st century and I can't abide their outmoded belief system. You need to ditch them for your own good. Disregard their marriage being successful (if it is). That's all luck and small sample size."

Maybe the parents don't approve of you for whatever reason, so they throw up roadblocks to dissuade you. "Hey, don't get married till you're done with law school. Then if that dude turns out to be a douche, you won't have to depend on him. And no cohabitating. Oh, and real men have a Rollie Fingers mustache and sing in a barbershop quartet, so he needs to get on that. If we think of anything else, we'll let you know." *Parents hi-five each other.* This presents a different challenge. Maybe you want to try to improve their opinion of you. Maybe you say to the gf, "Hey, I'd like you to stand up for me." Maybe you just stick around with your gf and they grudgingly accept you like Meathead from All in the Family.

Or, maybe they're evil control freaks binding her in chains of TEH RELIGION. Maybe they don't want the Joneses next door to know their daughter is a harlot, and that's more important to them than your relationship. Then you're fucked. Run.

Secondly, the gf. What does she believe is best for you guys? You've stating she's willing to live with you, but does she want to? Is she pro, con, or doesn't think it matters either way? Is she afraid of shaming/disappointing her parents due to her love/pride/respect for them? Asserting independence from parents is a big deal for people. Or is it strictly that she doesn't want to be disinherited? There's a significant difference in your ages (for me, anyway. I was much dumber at 24 than 29, but it's different for everyone), and I'm guessing in your life experience as well. So maybe she wants to be able to stand on her own two feet so she goes into marriage with you as an equal and not a junior partner. Maybe she has a religious complex from her upbringing. Any of these possibilities require a different response from the both of you.

Thirdly, you. Why is this so important to you (not saying it should or it shouldn't be, just that it is and you should think about the reason why)? I don't think it's that you want to determine your compatibility because you already proposed, she accepted, and you have a date set. To me, that says you think the two of you are compatible. In ages of yore (eg. when her parents got married), not living together worked out alright often enough. What are your expectations of cohabitating? If she leaves hair in the drain, is that a deal breaker? Do you expect to discover she's a member of the Aryan Nation? Maybe you don't see the sense in paying double for rent when you're broke and going to be married in a year anyway. Or maybe you really love her and want to marry her, but are just a liiiitle bit afraid and need a small bit of affirmation to seal the deal for you. Or maybe your belief structure dictates that living together is a necessary step that civilized people perform on the path to marriage. You've said this is a potential deal breaker, but you haven't said why. That's the more important part.

Staying married requires you to have a good system in place to navigate this sort of thing. You need to be able to constructively get your viewpoint across. In your OP, it sounds like you have the opinion, "I'm right, she needs to man up, meh parents." To me it looks like there's multiple actors behaving irrationally (yourself included). Also, you seem to be conflating compromise to one party giving in to the other.

Anyways, I'd suggest you first figure out your rationale behind your position. What are the positives for your future marriage if you cohabitate? Figure out what's going on in your head. Once you've got that down, go to your gf and talk through her deal. Explain your position. Figure out what she thinks and why she thinks it. The disinherited thing sounds like a smoke screen to me (based on sister's marriage). If she puts you off, maybe talk about how and why you came to the conclusion you did, and why it's important to you. Then explain you want to understand where she's coming from as marriage is srs bzns and married couples deal with this stuff all the time. She might not entirely understand the reasons for her own feelings. When you're both on the same page, if necessary go to her parents. If she likes her parents, show them (and their beliefs) respect. After all, they raised your fiancee so they got something right at some point, even if only by accident.

Compromises - If you guys decide to go ahead and move in together and her parents are dead set against it, maybe throw them a face saving bone. Maintain two apartments, do charity work (that son-in-law shacked up with our daughter, but he works in the church soup kitchen every Monday and Rev Doasisay loves him so whaddaya gonna do), that sort of thing. If gf is against it, maybe start small and escalate. Ask her if you can stay at her place on weekends, and arrange to spend quality time with her in that period. See how that works out, and go from there. Anyways, there's no one right solution. You just need to find something that's workable for the two of you. The parents are irrelevant except insofar as they affect your gf.

Things NOT to do - Don't go to her religious leader (unless solely for the purpose of understanding gf's/parents beliefs). Doing this as an end around of the parents shows you don't respect them. You may not, but don't show it. Even if the dude sides with you, dropping this kind of GOTCHA! on the parents is not going to end well.

Don't try to be 'right'. It doesn't matter if you are and it'll leave the other party feeling bitter towards you. All that matters is that you and gf find a happy medium regardless of how irrational. Explaining to people how stupid they are is seldom a good way of persuading them to your point of view.

Don't be a doormat/ignore it/decide unilaterally. That'll blow up in your face down the road.

Don't take her for granted. Everyone says this, but when you're living with someone for an extended period, it's really, really hard to put into practice. This works both ways, too. Don't let her take you for granted.

Don't ask for marriage advice on TL. Seriously, there's maybe 6 or 7 worthwhile posts in thread, and the rest are awful. I'd guess the majority here are not married, and have not ever seriously considered it in an adult way. People on TL cannot grasp that other people are not themselves and can value different things.