It's just societal norms. A lot of people expect men to be men (laborer, breadwinner, and protector of the family) while women act like women (child raising, housework, socialization, etc). Some will call it "sexist," and others will say it's just an adherence to basic American standards that have worked well for generations.
For male docs, the dating pool gets bigger and bigger as they go through training and into their career. What woman doesn't want to date a rich, smart, and successful guy... and bring that guy, who has his life together, over to meet her family? A male doc/resident can date female professionals, or he could date the junior college girl working at the mall GAP store. Nobody really cares; he's the provider and worker of the family regardless of who his partner is.
For female docs (esp the poor ones who decide to be surgeons), the dating pool gets smaller and smaller as they progress through. Not only do they have less time to date/excercise/primp, but they face a ticking biological clock, increasing % of men their age "taken" (or dating younger women), and they will almost surely be questioned if they date someone who makes less money and/or has a less prestigous job than they do. If that's not enough, about 80% of hospital employees are women, so it's tough for female docs to meet a partner at the workplace (and likewise very easy for male docs to get dates from the company ink). Finally, any vacation, ma/paternity leave, etc is viewed as lazy in a hardworking profession like medicine, and female docs are therefore gonna have to accept being past their prime physical attractiveness and reproductive peak years by the time they finish training... or usually be labeled as "slacker" or "a cow" at work if they do take time off for family, having kids, etc.
The female docs are basically limited to dating/marrying medical colleagues, lawyers, MBAs, PhDs, grad engineers, pro athletes/entertainers, etc... or being questioned for "dating down." To compound things, it's sorta hard for many female docs to attract those kind of successful guys since (let's face it) most women who are highly attractive (ie the type most professional men date) didn't have to work hard, go to med school, and do a residency to find success in life. Most attractive women spent their high school and college years dating, partying, and jersey chasing... not in the library curled up with a biology textbook. I'm not saying single female docs aren't attractive sometimes, but we have to admit that a 70hr work week isn't exactly conducive to fitting in makeup in the morning, five workouts per week, manicures, body detailing, fashion shopping, tanning, and hair appointments that a lot of women with more traditional careers might fit in.
Also, regardless of how she looks, most (non-whipped) guys conform to American standards and just aren't comfortable with the doc woman "wearing the pants" in terms of household income, etc. No guy with a sense of self worth wants to be Mr. Mom and get nonstop jokes from his buddies while his wife, a female doc, is gone all day earning the family's money. However, for many women of all types, the opposite (aka "traditional") setup of a rich husband working while they raise the kids is a great life they dream about all along, though. You can accept it or hate it... but you can't change the weather, folks.