Some people turn gay.
Sometimes people do change their sexuality later in life. For various reasons. It's not like they just "discovered," that they were gay, but rather, over time, their preferences shifted. Human sexuality is a very complex thing, and it is very harmful to believe that people are either born gay, straight, or something else. In fact, there is no biological definition of "gay" or "straight." It is all about behaviour.
Now I am not saying that you can "force" someone to be straight overnight, or that it is easy as just thinking about something else. But the whole gay rights movement was founded on the idea that gay people "can't help it..." What if some people felt as if they did find the appeal in being gay, later in life? That could have been the case for Frank. I don't think that makes the gay movement somehow invalidated. Everyone has the right to their own preferences. It's not like being a "straight man" somehow makes you categorically attracted to every "woman" on the planet.
And when you mix transgender people and other things into the equation, sexuality becomes much more complicated... The point being, is that people's sexual preferences do change over time, and it is not somehow wrong or necessarily genetically hardwired.