I know I tend to be the contrarian here because I don't quite get the whole cuck thing - even though, I have to admit, I play just a bit of a female cuck for my own GF....
If you step outside the environment here, which is one of those internet hothouses where the unusual (or worse) may seem normal because everyone HERE is into it, it worries me that you seem so focused if not even a bit obsessed about getting your fiancee' and soon to be wife into it!!! You have a woman who loves you and wants to spend her whole life with you, and is NOT interested in other men - I know plenty of girls and guys, including in relationships and marriages, who would ******* for that their partner to be that exclusively-minded!!!
In terms of practical suggestions...and thanks to those who've chipped in with them...I agree with the others who've already suggested letting this drop until after you're married, and the relationship has matured more. If your girl's at all into toys - and it sounds like she might just not be too adventuresome, unfortunately - you could start off trying to get her to play with some smaller black toys, dildos or even just anal plugs (a lot of very useful toys like that, come in a choice of pink or black, especially if you check online, and black is a better matching color for anal play anyway - did I just sound like an interior designer, LOL?!). If she's really not adventuresome, and not even into experimenting with a bit of anal play, then you're stuck - and that may be a sign that kinkier things are not going to fly, either. If it was me it was me - and I do have some experience corrupting girls to my kinky ways and plans
- I'd try the small anal toys for a bit of stimulation...which girls usually like...and then work up to trying simulated double penetration with a large anal plug or small black dildo, telling her that you like it when it's really tight (get big enough back there, and it does press against the vaginal tunnel)...which girls often like, too, I can tell you that being stuffed back there makes a dick or a dildo hit my G-spot all that much harder, and sends me...which is also what a BBC by itself does
Maybe you can slowly work up to the idea...and the pleasure...or maybe not, and again, that itself will tell you something.
In the bigger picture, and given the present reality, I'll say that if your girl was into this, I'd have your back - though I'd warn you to be just a bit careful. Given that she's not, I would suggest you consider talking to someone about why you're so focused if not obsessed, about pushing something that could damage your relationship and marriage - or worse.
Lanie
p.s. It's also a bit misleading if your username is for a couple, but it's really just you. Have I started to piss you the fuck off yet?