Well... organised religion at its core is about (and relies on) living in denial, so I suppose, if you can get someone to the point where they can convince themselves that organised religion is truth, then it's going to be pretty simple for that person to also delude themselves about their sexuality.
I'm carefully using the term "organised religion" here as really that's what we're talking about. Very little to do with the philosophical concept of God.
But yes, for anyone who believes in the validity of organised religion at all, you sort of have to believe that they can indeed "reverse" homosexuality, at least to the same extent of belief as the person would have in the religion itself.
Though in a purely objective sense, of course, the person is still gay and the religion still "false"... unless we start delving into the depths of ontology and metaphysics where you could argue that the delusion
becomes the reality, at least for as long as it is a genuinely held belief.