Another concern for me is that I never really saw that "sparkle" in her eye for me. Maybe that sounds ridiculous, but I have seen that sparkle in many girls' eyes when interacting with me, and it almost always = sexual and romantic interest. And more concerningly, I have seen that sparkle in Sue's eyes for several other guys....I'm with Athol on this one and I would put it even more strongly. Ditch her and ditch her now. She's settling because he's just about finished med school and appears to be capable of keeping her in the style to which she would like to become accustomed. This is why it is pointless to convince a woman into a relationship; ultimately, she'd rather be in one with someone else.
The question I face now is: Cut my losses and move on, happy to escape a situation that probably won't work long term (due to lack of animal/primal attraction on her part)? Or, try to make this work, and continue to increase my social/sexual rank, knowing I've got a trustworthy and good girl, who's only real downfall is that she nearly certainly prefers more of an "asshole"/dominant guy than I am naturally?
As other commenters have pointed out, his value is only going to go up, not only relative to hers but to other women in general. So, he'd be compounding his mistake by permitting her to settle with an early investment in a higher status man whose status she won't value. It would be fine, in fact, perhaps even ideal if she was truly in love with him, but it is readily apparent that she isn't.
Move on. Look elsewhere. Be thankful and pleased that you figured this out on the right side of the marital ceremony. And don't forget, for highly educated men, there are now more girls on the girl tree than ever before. And then, of course, there is the small matter of the ultimatum. I think the doctor-to-be must have a mistaken conception of the term "sigma", because any genuine sigma would know there is only one correct response to an ultimatum and it begins with the letter "F". As in what she can go and do by herself, to herself.
To go ahead and marry a woman after being posed that sort of ultimatum wouldn't so much cement BETA status as marital bitch status. She could have asked nicely. She could have pleaded her case. She could have told him how important he was to her. But instead, she chose to run a power move on him. So, think about what that says concerning her probable approach to quotidian married life....