I think people are conflating a few things here. There are several fallacies being peddled by certain people, which are quite harmful. Those fallacies include:

1.) Sexuality and gender identity are choices ("sexual preference") for pretty much everyone.

2.) Sexuality and gender identity are inborn for pretty much everyone.

3.) Sexuality and gender identity are a combination of nature+nurture for pretty much everyone.

4.) Sexuality and gender identity are fluid and constantly-evolving for pretty much everyone.

The flaw of subscribing to any of these four fallacies -- and applying it to EVERYONE across-the-board --- is that they take a one-size-fits-all approach to analyzing other people's identities.

For some individuals, one of those fallacies might be true (as it pertains to them INDIVIDUALLY). For other individuals, any combination of those fallacies might simultaneously exist, or alternately exist at different points during their lives.

Crazy thing, that INDIVIDUALITY, isn't it?