It depends on their criterion for "proven otherwise". If it's impossible for somebody to be proven straight to them, then possibly.
You see by your criterion, the set of "people who are alive" and "people who are gay" are exactly the same, making the words, in some ways, indistinguishable from each other.
---
Myself, I tend to take people's word on such things. If you say you're gay, I believe you. If you say you're not, I believe you.
Usually.