I sort of see what you're saying. However, I would say that perhaps if its so important for this hypothetical anti-gay therapist to avoid catching the gay disease
they should then choose to work in a large city where they have a referral base to work from, then your problem here is neatly solved.
On the other hand, lets say this anti-gay therapist instead insists on staying in a small community, and this therapist is agreeable to seeing patients on any emergency basis (of any kind), so they're not grossly negligent with their care provision.
Let's say this therapist is approached by some gay client with some existential issues, wants to work on personal growth, etc. - if that therapist doesn't want to work with that gay patient (I'm obviously playing devil's advocate here), then what's the problem? First, it's not an emergency issue. Second, that same client could easily seek out the many
practicioners available online which grow in number day by day who would be happy to work with them.
It may be f****d up that such a practicioner would do such a thing, and online therapy has many drawbacks from live therapy, but given all of this, again, what would be the harm, and why shouldn't that person be allowed to hang up a shingle?