I actually know quite a bit about this topic, so I am just going to say what I know/think. It gets kind of "nerdy".

I am quite familiar with research studies on the etiology ("causes") of sexual orientation (nothing conclusive, to say the least), and there is really no evidence of a causal (correlational means that two variables (traits/events/etc)) are linked, (causal means that one factor causes another).

For example, while LGBT folks tend to have higher rates of mental health issues, abuse, etc, there is NO evidence that mental illness/abuse CAUSES LGBT sexual orientation. Rather, it can be a result of prejudice, random occurrence, or there can be "third" variables (other events that influence both mental health, childhood abuse/neglect AND non-heterosexual sexual orientation).

(Note: I have a Ph.D. in a research heavy field, and I have read about sexual orientation out of personal interest, not as part of my professional activities.)

There is some research on "sexual fluidity" (changes of sexual orientation based on experience), in particular based on women. Different women have different levels of "fluidity" (roughly meaning ability for their sexual orientation to change based on circumstances). And yes, events in one's life can alter the trajectory of one's sexual orientation development. However, in NO way does that mean that one "chooses" it. Furthermore, they don't know what those events are - other than the classic example of higher levels of same-sex behavior reported in environments were folks are primarily exposed to members of the same-sex and NOT the opposite-sex.

If you read the policies from the American Psychological Association (governing body of professional psychologists) they state that sexual orientation is considered to be in a spectrum AND that, like with any other trait, there are multiple complex variations in sexual orientation that sometimes change over time.

They state there is NOTHING pathological with non-heterosexual orientations. Psychologists are not there to judge it OR try to change it. Their job, if appropriate, is to help the patient cope with any distress that may result from their sexual orientation (discrimination, internalized-homophobia, etc.)

That being said, it's important to think what you want from your therapist and whether she is able to provide it. If you want her to just accept you for who you are, just tell her that's why you brought it up, that you are not interested in exploring how SHE thinks you've developed your sexual orientation. If you want her help coping with internalized feelings of shame, then let her know.

A lot of therapists get minimal training in LGBT issues, unless it's a particular interest of theirs, or live in a place where it can almost not be avoided (San Francisco, Seattle, etc.). Even therapists in LGBT friendly places can have their own prejudices (and not be even aware of it!). At the same time, therapists without much LGBT issues training, can be great due to their personal experiences, etc.

I know it's a VERY tough conversation to have (I was terrified!), but when I came out to my therapist as bisexual, I was very direct in asking her, her opinions and expressing times when I thought she was being "insensitive". She was not defensive at all, and was very thoughtful about what I had to say. This was a good sign.