You could be describing me 20 years ago.
Bright high achiever, with long term bf (although mine had a job), religious with CofE background but more modern "happy clappy" church, did not want sex before marriage with then bf, engaged in early 20s with plans to marry a year later.
Except it was for stability, security and fear - exactly as mathanxiety's post reads. There was no spark, no buzz, nothing really. I thought I loved him but I suspect I was more in love with the idea of love, weddings, big dresses and church bells.
My parents weren't happy, they were very concerned about the risk of me abandoning career plans to become sahm / housewife. Which is a valid and right life choice for many people, but would not have been for me. Then bf assured them he would support me in any life choices.
The wedding was everything I had dreamed of. I started crying on the plane heading off on honeymoon thinking "omg what have I done..." The marriage lasted 4 months proper, 2 years ish after that. Then DH wanted me to drop out of training, be a wife & mother, stay home ...
Fast forward two decades and I am with DH2, two amazing boys, a wonderful marriage with the stability and reliability that I craved, together with much spark and more support than I could ever hope for.
I wouldn't have listened to my parents that first time though. But they supported me, Dad made our cake, they told me their worries but were 100% there for me. So at 4am when it all fell apart they were at the door to take me home when I called.
Tell your DD of your concerns but equally - if it doesn't work out - make sure she knows she can run to you if she needs to. To this day there has never been an "I told you so" or any mention of how my stubborn insistence on going ahead with it affected our family.
Oh and I'm agnostic now too. Once I'd got out of the marriage and away from the rather all embracing church environment (it was our entire social life) I questioned things too much to go back.