I'm getting so frustrated at this. I've done everything I could. My primary hero is a physician and the bride-to-be is an old patient of his. My mission objective is now to marry her, but no matter what I do, the "Ask hand in marriage" interaction is ALWAYS rejected. My physician has full focus, has two rings in his pocket and has 100% relationship with his courting partner. Why the hell does she keep rejecting him? The quest is going to expire soon and my performance is rapidly decreasing. I've already lost platinum rating and it's going down further. Help?!

Did you choose to marry at a location other than one of the churches? Because for some reason the sims will refuse to get married else where (routing issues I believe). I had chosen to get married in the forest on that quest and then had to cheat a little to actually get them married (went into the game files, turned testingcheats on, and then set the two as being married). I now know to avoid that choice if I do that quest again.
Or is it something else?

It turned out the bride was in a horrible mood all this time. Had my physician treat her for any ailments and then propose, worked. Thanks for your help though!

Actually, they can marry outside a church, it's just when you do the marriage interaction, they need to be positioned a certain way