Aproaches to cycling vary from person to person. Right now, you have a desire to ride a bike. I'm assuming your fiancee also does as well. If not then you probably don't want to push her into doing something she is vehemently opposed to. And you probably don't want to jump into this with performance road bikes right away either. My best advice is to find what's comfortable today that will allow you to enjoy cycling and go with that. That may be a hybrid or single-speed townie/breach cruiser or MTB or road bike. We don't know. You don't know. But it's something you owe to yourselves to find out by trying out different bikes.

Cycling is a sport you grow into... A full-on race-ready or even high performance road bike today may not be the most comfortable thing for you to ride. Eventually your riding style changes and your body changes too and in a year that road bike may be the most comfortable thing for the type of riding you want to do while the hybrid or converted MTB that might be comfortable to ride now may turn out to be limiting in the future. But you have to get there from here. So go find a bike shop and try out different styles of bikes with your fiancee. Then each of you determine what kind of bike and what kind of cycling suits you right now. And plan to buy something that will work for you for the next 6 months or so. If you're just starting out cycling (which given a lapse of even a couple of years is really how you should consider yourself) then you don't want to drop a ton of cash on a bike that you may grow apart from in relatively short order. You'd be surprised at how quickly your body acquaints itself to cycling.

As for your fiancee's fear of front brakes, I can understand her hesitation. If she's willing to ride a bike with both front and rear brakes and can restraint herself from using too much front brake then I would still suggest you attempt to convince her to look into such a bike. Most likely you will be travelling at slower speeds to start out with so relying only on the rear brake won't be too detrimental. Eventually she should learn and be encouraged to properly use the front brake. If she simply just can't bring herself to even get on a bike with a front brake then by all means try and find a bike with a coaster brake. This will of course limit your choices... especially as you graduate to the higher performance end of the cycling spectrum. But right now, it is more important to get her a bike she is comfortable riding and operating.