I'd really prefer not to get into this discussion, but I just feel compelled. First, being gay can certainly have a large impact on an individual's potential to be a good doctor. Often, gay men and women encounter discrimination and hate and show great determination and maturity in the face of adversity, two qualities I'm sure most would agree would affect anyone's potential to be a good doctor. Obviously, many people have similar experiences, and gay men and women have no monopoly on discrimination. Nevertheless, being gay does mean coming to terms with some hard lessons early on, lessons that could certainly affect someone's potential to be a doctor.
Second, the world assumes that an individual is straight as long as s/he doesn't correct that assumption. It's called heterosexual privilege - you can read about it if you want. Would you want adcoms assuming something about you that isn't true? While there's no "box" to check for being gay, it's certainly easy to work into an essay or your personal statement. For many GLBTQ individuals, being open about themselves is very important.
Finally, gay doctors are increasingly important in the medical world. There are certainly issues that I would feel more comfortable discussing with a gay doctor, being gay myself (Look at how subtlety I slipped that in there! Think I could do it in an essay?
😱 ). As HIV/AIDS and TB continue to rise in the GLBTQ population, gay doctors will become ever more important for everyone's general welfare.
Your post is ignorant, uncaring, and unsympathetic to the OP's original post, which was only asking a simple question. You clearly live in a heterocentric and narrow world. I hope GLBTQ patients never have to come to you for medical treatment.
To the OP, most schools have non-discrimination clauses. Check each individual school's clause in MSAR - most say specifically that they don't discriminate on the basis of sexual orientation. In terms of schools where being openly gay on your app might hurt you, the only one that comes to mind is Loma Linda, which is Christian. Good luck!