You actually missed the main point of my post -- my assertion that you don't know any gay people was just speculation, and I wonder how deep your relationship is with this person and if he knows what you actually think about his sexual orientation.
Do you think your beliefs are above criticism? That simply by saying something is a belief of yours, it is immune to backlash? Some people believe that vaccines cause autism. What if a doctor expressed that belief. Should I respect that opinion? Absolutely not. Because it would be bad for patients.
I think that anybody who thinks homosexuality is not a biological phenomenon is ignorant of the evidence. Such a fundamental misunderstanding of human sexuality is akin to a fundamental misunderstanding of human anatomy or physiology, especially if you are a primary care doc or a psychiatrist.
And Christian psychiatrist does not necessarily mean bigoted psychiatrist. Tons of Christians accept that homosexuality is not a choice, especially ones well-educated enough to be physicians. Millennials, as a generation, are also very accepting of this, even if they are religious.