To be fair, I think the newer research is showing that some indeterminate (yet possibly significant) percentage of people have fluid sexualities, which implies a lack of a static sexual orientation, but also implies that outside attempts to change this orientation are fruitless (therefore continuing to reject the notion of "therapeutic" changes of sexual orientation). The insistence that gay people couldn't change their orientation specifically developed from resistance to the idea that sexual attraction could be changed or fully repressed. I'm thinking it's more of a historical artifact and the first "line of defense" against conversion therapy crusaders, as most people, for some odd reason, don't find sexuality theory as fascinating as some of us. Even the presence of fluid sexuality doesn't necessarily imply that sexual orientation can be purposefully changed. People who report a fluid orientation don't suddenly wake up and say "Well, I think I'll be attracted to (insert gender here) for the rest of my life" - it literally is comparable to a fluid that has no fixed shape, but does have a fixed volume and consistency. I'd best describe it as a shifting of poles that happens pretty randomly; just because a person prefers men/women now doesn't always mean they will prefer that gender for the rest of their life, or even for the rest of any given length of time.
As a consequence of this, trying to convince people to pick a side usually doesn't work out. You're entirely right that the "nobody chooses..." argument was done out of political expediency, but it wasn't done because they knew deep down that the "conversioners" (for lack of a better term) were correct. They were choosing to (over)simplify areas of sexuality that weren't well-understood and remain that way to this day. Thankfully the conversation is starting to change as people grow more open to discussing this sort of thing.
