I think that some of the social power of viewing sexuality as an innate characteristic is to respond to discrimination and the work to add sexual orientation as a protected class (along with race, religion, etc).
There are other protected classes that are mutable like religion and creed, but the argument that discrimination based on sexuality should be illegal like race is stronger if it’s not something that people choose.
Of course discrimination should be minimized regardless of innate or not, but the loud arguments I’ve heard are about how sexuality is something born with. The whole idea of sexual fluidity, to me, means that there’s a whole malleability and I assume there’s some conscious ability to impact it.